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ISBN13: 9780131877153 ISBNl0: 01318771S1
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Applied Multivariate Statistical AnalysisI
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SIXTH EDITION
Applied Multivariate Statistical AnalysisRICHARD A. JOHNSONUniversity of WisconsinMadison
DEAN W. WICHERNTexas A&M University
PEARSON~
Prentice
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,brary of Congress CataloginginPublication Data
>hnson, Richard A. Statistical analysisiRichard A. Johnson.61h ed. Dean W. Winchern p.em. Includes index. ISBN 0131877151 1. Statistical Analysis~IP
Data Available
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2007 Pearson Education, Inc. Pearson Prentice Hall Pearson Education, Inc. Upper Saddle River, NJ 07458
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ISBN13: 9780131877153 0 13  187715' 1 ISBNl0:Pearson Education LID., London Pearson Education Australia P1Y, Limited, Sydney Pearson Education Singapore, Pte. Ltd Pearson Education North Asia Ltd, Hong Kong Pearson Education Canada, Ltd., Toronto Pearson Educaci6n de Mexico, S.A. de C.V. Pearson EducationJapan, Tokyo Pearson Education Malaysia, Pte. Ltd
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ContentsPREFACE
xv1
1
ASPECTS OF MULTlVARIATE ANALYSIS
1.1 1.21.3
Introduction 1 Applications of Multivariate Techniques 3 The Organization of Data 5Arrays,5 Descriptive Statistics, 6 Graphical Techniques, 11
1.4
Data Displays and Pictorial Representations 19Linking Multiple TwoDimensional Scatter Plots, 20 Graphs of Growth Curves, 24 Stars, 26 Chernoff Faces, 27
1.5
1.6
Distance 30 Final Comments 37 Exercises 37 References 4749
2
MATRIX ALGEBRA AND RANDOM VECTORS
2.1 2.2
Introduction 49 Some Basics of Matrix and Vector Algebra 49Vectors, 49 Matrices, 54
2.3 2.4 2.5 2.6
Positive Definite Matrices 60 A SquareRoot Matrix 65 Random Vectors and Matrices 66 Mean Vectors and Covariance Matrices
68
Partitioning the Covariance Matrix, 73 The Mean Vector and Covariance Matrix for Linear Combinations of Random Variables, 75 Partitioning the Sample Mean Vector and Covariance Matrix, 77
2.7
Matrix Inequalities and Maximization 78vii
viii
Contents
Contents
ix
Supplement 2A: Vectors and Matrices: Basic Concepts 82Vectors, 82 Matrices, 87
5
INFERENCES ABOUT A MEAN VECTOR
210
Exercises 103 References 1103SAMPLE GEOMETRY AND RANDOM SAMPLING
5.1 5.2 5.3 111 5.4
Introduction 210 The Plausibility of Po as a Value for a Normal Population Mean 210 HotelIing's T2 and Likelihood Ratio Tests 216General Likelihood Ratio Method, 219
3.1 3.2 3.3 3.4
Introduction 111 The Geometry of the Sample 111 Random Samples and the Expected Values of the Sample Mean and Covariance Matrix 119 Generalized Variance 123Situations in which the Generalized Sample Variance Is Zero, 129 Generalized Variance Determined by I R I and Its Geometrical Interpretation, 134 Another Generalization of Variance, 137
Confidence Regions and Simultaneous Comparisons of Component Means 220Simultaneous Confidence Statements, 223 A Comparison of Simultaneous Confidence Intervals with OneataTime Intervals, 229 The Bonferroni Method of Multiple Comparisons, 232
5.5 5.6
Large Sample Inferences about a Population Mean Vector Multivariate Quality Control Charts 239
234
3.5 3.6
Sample Mean, Covariance, and Correlation As Matrix Operations 137 Sample Values of Linear Combinations of Variables Exercises 144 References 148
140
Charts for Monitoring a Sample of Individual Multivariate Observations for Stability, 241 Control Regions for Future Individual Observations, 247 Control Ellipse for Future Observations 248 2 ' T Chart for Future Observations, 248 Control Charts Based on Subsample Means, 249 Control Regions for Future SUbsample Observations, 251
4
THE MULTlVARIATE NORMAL DISTRIBUTION
149
4.1 4.2
Introduction 149 The Multivariate Normal Density and Its Properties 149Additional Properties of the Multivariate Normal Distribution, 156
IJ
5.7 5.8
I
4.3
Sampling from a Multivariate Normal Distribution and Maximum Likelihood Estimation 168The Multivariate Normal Likelihood, 168 Maximum Likelihood Estimation of P and I, 170 Sufficient Statistics, 173
Inferences about Mean Vectors when Some Observations Are Missing 251 Difficulties Due to TIme Dependence in Multivariate Observations 256 Supplement 5A: Simultaneous Confidence Intervals and Ellipses as Shadows of the pDimensional Ellipsoids 258 Exercises 261 References 272273
6
COMPARISONS OF SEVERAL MULTIVARIATE MEANS
4.4 4.5 4.6
The Sampling Distribution of X and S 173Properties of the Wishart Distribution, 174
6.1 6.2
Introduction 273 Paired Comparisons and a Repeated Measures Design 273Paired Comparisons, 273 A Repeated Measures Design for Comparing Treatments, 279
LargeSample Behavior of X and S 175 Assessing the Assumption of Normality 177Evaluating the Normality of the Univariate Marginal Distributions, 177 Evaluating Bivariate Normality, 182
6.3
Comparing Mean Vectors from Two Populations 284Assumptions Concerning the Structure of the Data, 284 Further Assumptions When nl and n2 Are Small, 285 Simultaneous Confidence Intervals, 288 The TwoSample Situation When 1:1 oF l;z,291 An Approximation to the Distribution of T2 for Normal Populations When Sample Sizes Are Not Large, 294
4.7 4.8
Detecting Outliers and Cleaning Data 187Steps for Detecting Outtiers, 189
Transformations to Near Normality 192Transforming Multivariate Observations, 195
Exercises 200 References 208
6.4
Comparing Several Multivariate Population Means (OneWay Manova) 296Assumptions about the Structure of the Data for OneWay MANOVA, 296
x
ContentsA Summary of Univariate ANOVA, 297 Multivariate Analysis of Variance (MANOVA), 301
Contents8 PRINCIPAL COMPONENTS
xi
430
6.5 6.6 6.7
Simultaneous Confidence Intervals for Treatment Effects 308 Testing for Equality of Covariance Matrices 310 1\voWay Multivariate Analysis of Variance 312Univariate TwoWay FixedEffects Model with Interaction, 312 Multivariate Two Way FixedEffects Model with Interaction, 315
8.1 8.2
Introduction 430 Population Principal Components 430Principal Components Obtained from Standardized Variables 436 Principal Components for Covariance Matrices ' with Special Structures, 439
6.8 6.9 6.10
Profile Analysis 323 Repeated Measures Designs and Growth Curves 328 Perspectives and a Strategy for Analyzing Multivariate Models 332 Exercises 337 References 358360
8.3
Summarizing Sample Variation by Principal Components 441The Number of Principal Components, 444 Interpretation of the Sample Principal Components, 448 Standardizing the Sample Principal Components, 449
8.4 8.5
Large Sample Properties of Aj and ej, 456 Testing for the Equal Correlation Structure, 457
Graphing the Principal Components 454 Large Sample Inferences 456
7
MULTlVARIATE LINEAR REGRESSION MODELS
8.6
Monitoring Quality with Principal Components 459Checking a Given Set of Measurements for Stability, 459 Controlling Future Values, 463
7.1 7.2 7.3
Introduction 360 The Classical Linear Regression Model 360 Least Squares Estimation 364SumoJSquares Decomposition, 366 Geometry of Least Squares, 367 Sampling Properties of Classical Least Squares Estimators, 369
Supplement 8A: The Geometry of the Sample Principal Component Approximation 466The pDimensional Geometrical Interpretation, 468 The nDimensional Geometrical Interpretation, 469
7.4 7.5 7.6
Inferences About the Regression Model 370Inferences Concerning the Regression Parameters, 370 Likelihood Ratio Tests for the Regression Parameters, 374
Exercises 470 References 4809 FACTOR ANALYSIS AND INFERENCE FOR STRUCTURED COVARIANCE MATRICES
Inferences from the Estimated Regression Function 378Estimating the Regression Function at Zo, 378 Forecasting a New Observation at Zo, 379
481
Model Checking and Other Aspects of Regression 381Does the Model Fit?, 381 Leverage and Influence, 384 Additional Problems in Linear Regression, 384
9.1 9.2 9.3
Introduction 481 The Orthogonal Factor Model 482 Methods of Estimation 488The Pri,!cipal Component (and Principal Factor) Method, 488 A ModifiedApproachthe Principal Factor Solution, 494 The Maximum Likelihood Method, 495 A Large Sample Test for the Number of Common Factors 501
7.7
Multivariate Multiple Regression 387Likelihood Ratio Tests for Regression Parameters, 395 Other Multivariate Test Statistics, 398 Predictions from Multivariate Multiple Regressions, 399
9.4 9.5
Factor Rotation
504
'
7.8 7.9 7.10
The Concept of Linear Regression 401Prediction of Several Variables, 406 Partial Correlation Coefficient, 409
Oblique Rotations, 512
Factor Scores 513The Weighted Least Squares Method, 514 The Regression Method, 516
Comparing the Two Formulations of the Regression Model 410Mean Corrected Form of the Regression Model, 410 Relating the Formulations, 412
9.6
Multiple Regression Models with Time Dependent Errors 413 Supplement 7A: The Distribution of the Likelihood Ratio for the Multivariate Multiple Regression Model 418 Exercises  420 References 428
Perspectives and a Strategy for Factor Analysis 519 Supplement 9A: Some Computational Details for Maximum Likelihood Estimation 527Recommended Computational Scheme, 528 Maximum Likelihood Estimators of p = L.L~
+ 1/1. 529
Exercises 530 References 538
xii
Contents10CANONICAL CORRELATION ANALYSIS
Contents
xiii
539
10.1 10.2 10.3
Introduction 539 Canonical Variates and Canonical Correlations 539 Interpreting the Population Canonical Variables 545Identifying the {:anonical Variables, 545 Canonical Correlations as Generalizations of Other Correlation Coefficients, 547 The First r Canonical Variables as a Summary of Variability, 548 A Geometrical Interpretation of the Population Canonical Correlation Analysis 549 .
Testing for Group Differences, 648 Graphics, 649 Practical Considerations Regarding Multivariate Normality, 649
Exercises 650 References 66912 CLUSTERING, DISTANCE METHODS, AND ORDINATION
671
12.1 12.2
Introduction 671 Similarity Measures 673
.
10.4 10.5
The Sample Canonical Variates and Sample Canonical Correlations 550 Additional Sample Descriptive Measures 558Matrices of Errors ofApproximations, 558 Proportions of Explained Sample Variance, 561
Distances and Similarity Coefficients for Pairs of Items, 673 Similarities and Association Measures for Pairs of Variables, 677 Concluding Comments on Similarity, 678
12.3
Hierarchical Clustering Methods
680
10.6
Large Sample Inferences 563 Exercises 567 References 574
Single Linkage, 682 Complete Linkage, 685 Average Linkage, 690 Ward's Hierarchical Clustering Method, 692 Final CommentsHierarchical Procedures, 695
11
DISCRIMINATION AND CLASSIFICATION
575
12.4
Nonhierarchical Clustering Methods 696Kmeans Method, 696 Final CommentsNonhierarchical Procedures, 701
11.1 11.2 11.3
Introduction 575 Separation and Classification for Two Populations 576 Classification with 1\vo Multivariate Normal Populations 584Classification of Normal Populations When l:1 = l:z = :,584 Scaling, 589 Fisher's Approach to Classification with 1Wo Populations, 590 Is Classification a Good Idea?, 592 Classification of Normal Populations When:1 =F :z, 593
12.5 12.6 12.7
Clustering Based on Statistical Models 703 Multidimensional Scaling 706The Basic Algorithm, 708 .
Correspondence Analysis 716Algebraic Development of Correspondence Analysis, 718 Inertia,725 Interpretation in Two Dimensions, 726 Final Comments, 726
11.4 11.5
Evaluating Classification Functions 596 Classification with Several Populations 606The Minimum Expected Cost of Misclassification Method, 606 Classification with Normal Populations, 609
12.8 12.9
Biplots for Viewing Sampling Units and Variables 726Constructing Biplots, 727
11.6
Fisher's Method for Discriminating among Several Populations 621Using Fisher's Discriminants to Classify Objects, 628
Procrustes Analysis: A Method for Comparing Configurations 732Constructing the Procrustes Measure ofAgreement, 733
Supplement 12A: Data Mining 740Introduction, 740 The Data Mining Process, 741 Model Assessment, 742
11.7
Logistic Regression and Classification 634Introduction, 634 The Logit Model, 634 Logistic Regression Analysis, 636 Classification, 638 Logistic Regression with Binomial Responses, 640
Exercises 747 References 755APPENDIX
11.8
Final Comments
644
Including Qualitative Variables, 644 Classification Trees, 644 Neural Networks, 647 Selection of Variables, 648
757 764 767
DATA INDEXSUBJECT INDEX
:l:
,
Preface
Iif!
INTENDED AUDIENCE
j
I
This book originally grew out of our lecture notes for an "Applied Multivariate Analysis" course offered jointly by the Statistics Department and the School of Business at the University of WisconsinMadison. Applied Multivariate StatisticalAnalysis, Sixth Edition, is concerned with statistical methods for describing and analyzing multivariate data. Data analysis, while interesting with one variable, becomes truly fascinating and challenging when several variables are involved. Researchers in the biological, physical, and social sciences frequently collect measurements on several variables. Modem computer packages readily provide the numerical results to rather complex statistical analyses. We have tried to provide readers with the supporting knowledge necessary for making proper interpretations, selecting appropriate techniques, and understanding their strengths and weaknesses. We hope our discussions wiII meet the needs of experimental scientists, in a wide variety of subject matter areas, as a readable introduction to the statistical analysis of multivariate observations.
LEVEL
r
I
Our aim is to present the concepts and methods of muItivariate analysis at a level that is readily understandable by readers who have taken two or more statistics courses. We emphasize the applications of multivariate methods and, consequently, have attempted to make the mathematics as palatable as possible. We avoid the use of calculus. On the other hand, the concepts of a matrix and of matrix manipulations are important. We do not assume the reader is familiar with matrix algebra. Rather, we introduce matrices as they appear naturally in our discussions, and we then show how they simplify the presentation of muItivariate models and techniques. The introductory account of matrix algebra, in Chapter 2, highlights the more important matrix algebra results as they apply to multivariate analysis. The Chapter 2 supplement provides a summary of matrix algebra results for those with little or no previous exposure to the subject. This supplementary material helps make the book selfcontained and is used to complete proofs. The proofs may be ignored on the first reading. In this way we hope to make the book accessible to a wide audience. In our attempt to make the study of muItivariate analysis appealing to a large audience of both practitioners and theoreticians, we have had to sacrifice xv
xvi
Preface
Preface
xvii
The resulting presentation IS rather SUCCInct and difficult the fIrst ~Ime throu~h. We hope instructors will be a?le to compensat.e for the une~enness In l~vel by JUdiciously choosing those s~ctIons, and subsectIOns, appropnate for theIr students and by toning them tlown If necessary.
~~ve summarized a volumi?ous amount .of materi~l?n regres~ion ~n Chapter 7.
onsistency of level. Some sections are harder than others. In particular, we
agrams and verbal descriptions to teach the corresponding theoretical developments. If the students have uniformly strong mathematical backgrounds, much of the book can successfully be covered in one term. We have found individual dataanalysis projects useful for integrating material from several of the methods chapters. Here, our rather complete treatments of multivariate analysis of variance (MANOVA), regression analysis, factor analysis, canonical correlation, discriminant analysis, and so forth are helpful, even though they may not be specifically covered in lectures.CHANGES TO THE SIXTH EDITION
ORGANIZATION AND APPROACH
The methodological "tools" of multlvariate analysis are contained in Chapters 5 through 12. These chapters represent the heart of the book, but they cannot be assimilated without much of the material in the introd~ctory Chapters 1 thr?~gh 4. Even those readers with a good kno~ledge of matrix algebra or those willing t accept the mathematical results on faIth should, at the very least, peruse Chapo 3 "Sample Geometry," and Chapter 4, "Multivariate Normal Distribution." ter , Our approach in the methodological ~hapters is to ~eep the discussion.dit and uncluttered. Typically, we start with a formulatIOn of the population re~dels delineate the corresponding sample results, and liberally illustrate every:'ing ~ith examples. The exa~ples are of two types: those that are simple and hose calculations can be easily done by hand, and those that rely on realworld ~ata and computer software. These will provide an opportunity to (1) duplicate our analyses, (2) carry out the analyses dictated by exercises, or (3) analyze the data using methods other than the ones we have used or suggest~d. . The division of the methodological chapters (5 through 12) Into three umts llo~s instructors some flexibility in tailoring a course to their needs. Possible a uences for a onesemester (two quarter) course are indicated schematically. seq . . . fr h t Each instructor will undoubtedly omit certam sectIons om some c ap ers to cover a broader collection of topics than is indicated by these two choices.Getting Started Chapters 14
New material. Users of the previous editions will notice several major changes in the sixth edition.
Twelve new data sets including national track records for men and women, psychological profile scores, car body assembly measurements, cell phone tower breakdowns, pulp and paper properties measurements, Mali family farm data, stock price rates of return, and Concho water snake data. Thirty seven new exercises and twenty revised exercises with many of these exercises based on the new data sets. Four new data based examples and fifteen revised examples. Six new or expanded sections:1. Section 6.6 Testing for Equality of Covariance Matrices
2. Section 11.7 Logistic Regression and Classification 3. Section 12.5 Clustering Based on Statistical Models 4. Expanded Section 6.3 to include "An Approximation to the, Distribution of T2 for Normal Populations When Sample Sizes are not Large" 5. Expanded Sections 7.6 and 7.7 to include Akaike's Information Criterion 6. Consolidated previous Sections 11.3 and 11.5 on two group discriminant analysis into single Section 11.3Web Site. To make the methods of multivariate analysis more prominent in the text, we have removed the long proofs of Results 7.2,7.4,7.10 and 10.1 and placed them on a web site accessible through www.prenhall.comlstatistics. Click on "Multivariate Statistics" and then click on our book. In addition, all full data sets saved as ASCII files that are used in the book are available on the web site. Instructors' Solutions Manual. An Instructors Solutions Manual is available on the author's website accessible through www.prenhall.comlstatistics.For information on additional forsale supplements that may be used with the book or additional titles of interest, please visit the Prentice Hall web site at www.prenhall. corn.
For most students, we would suggest a quick pass through the first four hapters (concentrating primarily on the material in Chapter 1; Sections 2.1, 2.2, ~.3, 2.5, 2.6, and 3.6; and the "assessing normality" material in Chapter ~) followed by a selection of methodological topics. For example, one mIght dISCUSS the comparison of mean vectors, principal components, factor analysis, discriminant analysis and clustering. The di~cussions could feature the many "worke? out" examples included in these sections of the text. Instructors may rely on dI
cs""iii
Preface
,ACKNOWLEDGMENTSWe thank many of our colleagues who helped improve the applied aspect of the book by contributing their own data sets for examples and exercises. A number of individuals helped guide various revisions of this book, and we are grateful for their suggestions: Christopher Bingham, University of Minnesota; Steve Coad, University of Michigan; Richard Kiltie, University of Florida; Sam Kotz, George Mason University; Him Koul, Michigan State University; Bruce McCullough, Drexel University; Shyamal Peddada, University of Virginia; K. Sivakumar University of Illinois at Chicago; Eric Smith, Virginia Tecn; and Stanley Wasserman, University of Illinois at Urbanaciiampaign. We also acknowledge the feedback of the students we have taught these past 35 years in our applied multivariate analysis courses. Their comments and suggestions are largely responsible for the present iteration of this work. We would also like to give special thanks to Wai K wong Cheang, Shanhong Guan, Jialiang Li and Zhiguo Xiao for their help with the calculations for many of the examples. We must thank Dianne Hall for her valuable help with the Solutions Manual, Steve Verrill for computing assistance throughout, and Alison Pollack for implementing a Chernoff faces program. We are indebted to Cliff GiIman for his assistance with the multidimensional scaling examples discussed in Chapter 12. Jacquelyn Forer did most of the typing of the original draft manuscript, and we appreciate her expertise and willingness to endure cajoling of authors faced with publication deadlines. Finally, we would like to thank Petra Recter, Debbie Ryan, Michael Bell, Linda Behrens, Joanne Wendelken and the rest of the Prentice Hall staff for their help with this project. R. A. lohnson [email protected] D. W. Wichern [email protected]
Applied Multivariate Statistical Analysis
Chapter
ASPECTS OF MULTIVARIATE ANALYSIS1.1 IntroductionScientific inquiry is an iterative learning process. Objectives pertaining to the explanation of a social or physical phenomenon must be specified and then tested by gathering and analyzing data. In turn, an analysis of the data gathered by experimentation or observation will usually suggest a modified explanation of the phenomenon. Throughout this iterative learning process, variables are often added or deleted from the study. Thus, the complexities of most phenomena require an investigator to collect observations on many different variables. This book is concerned with statistical methods designed to elicit information from these kinds of data sets. Because the data include simultaneous measurements on many variables, this body .of methodology is called multivariate analysis. The need to understand the relationships between many variables makes multivariate analysis an inherently difficult subject. Often, the human mind is overwhelmed by the sheer bulk of the data. Additionally, more mathematics is required to derive multivariate statistical techniques for making inferences than in a univariate setting. We have chosen to provide explanations based upon algebraic concepts and to avoid the derivations of statistical results that require the calculus of many variables. Our objective is to introduce several useful multivariate techniques in a clear manner, making heavy use of illustrative examples and a minimum of mathematics. Nonetheless, some mathematical sophistication and a desire to think quantitatively will be required. Most of our emphasis will be on the analysis of measurements obtained without actively controlling or manipulating any of the variables on which the measurements are made. Only in Chapters 6 and 7 shall we treat a few experimental plans (designs) for generating data that prescribe the active manipulation of important variables. Although the experimental design is ordinarily the most important part of a scientific investigation, it is frequently impossible to control the
2 Chapter 1 Aspects of Multivariate Analysis generation of appropriate data in certain disciplines. (This is true, for example, in business, economics, ecology, geology, and sociology.) You should consult [6] and [7] for detailed accounts of design principles that, fortunately, also apply to multivariate situations. It will become increasingly clear that many multivariate methods are based upon an underlying proBability model known as the multivariate normal distribution. Other methods are ad hoc in nature and are justified by logical or commonsense arguments. Regardless of their origin, multivariate techniques must, invariably, be implemented on a computer. Recent advances in computer technology have been accompanied by the development of rather sophisticated statistical software packages, making the implementation step easier. Multivariate analysis is a "mixed bag." It is difficult to establish a classification scheme for multivariate techniques that is both widely accepted and indicates the appropriateness of the techniques. One classification distinguishes techniques designed to study interdependent relationships from those designed to study dependent relationships. Another classifies techniques according to the number of populations and the number of sets of variables being studied. Chapters in this text are divided into sections according to inference about treatment means, inference about covariance structure, and techniques for sorting or grouping. This should not, however, be considered an attempt to place each method into a slot. Rather, the choice of methods and the types of analyses employed are largely determined by the objectives of the investigation. In Section 1.2, we list a smaller number of practical problems designed to illustrate the connection between the choice of a statistical method and the objectives of the study. These problems, plus the examples in the text, should provide you with an appreciation of the applicability of multivariate techniques acroSS different fields. The objectives of scientific investigations to which multivariate methods most naturally lend themselves include the following: L Data reduction or structural simplification. The phenomenon being studied is represented as simply as possible without sacrificing valuable information. It is hoped that this will make interpretation easier. 2. Sorting and grouping. Groups of "similar" objects or variables are created, based upon measured characteristics. Alternatively, rules for classifying objects into welldefined groups may be required. 3. Investigation of the dependence among variables. The nature of the relationships among variables is of interest. Are all the variables mutually independent or are one or more variables dependent on the others? If so, how? 4. Prediction. Relationships between variables must be determined for the purpose of predicting the values of one or more variables on the basis of observations on the other variables. 5. Hypothesis construction and testing. Specific statistical hypotheses, formulated in terms of the parameters of multivariate populations, are tested. This may be done to validate assumptions or to reinforce prior convictions. We conclude this brief overview of multivariate analysis with a quotation from F. H. C Marriott [19], page 89. The statement was made in a discussion of cluster analysis, but we feel it is appropriate for a broader range of methods. You should keep it in mind whenever you attempt or read about a data analysis. It allows one to
Applications of Multivariate Techniques 3 maintain a proper perspective and not be overwhelmed by the elegance of some of the theory:If the results disagree with informed opinion, do not admit a simple logical interpreta
tion, and do not show up clearly in a graphical presentation, they are probably wrong. There is no magic about numerical methods, and many ways in which they can break down. They are a valuable aid to the interpretation of data, not sausage machines automatically transforming bodies of numbers into packets of scientific fact.
1.2 Applications of Multivariate TechniquesThe published applications of multivariate methods have increased tremendously in recent years. It is now difficult to cover the variety of realworld applications of these methods with brief discussions, as we did in earlier editions of this book. However, in order to give some indication of the usefulness of multivariate techniques, we offer the following short descriptions_of the results of studies from several disciplines. These descriptions are organized according to the categories of objectives given in the previous section. Of course, many of our examples are multifaceted and could be placed in more than one category.
.f
t
Data reduction or simplification
I
Using data on several variables related to cancer patient responses to radiotherapy, a simple measure of patient response to radiotherapy was constructed. (See Exercise 1.15.) ltack records from many nations were used to develop an index of performance for both male and female athletes. (See [8] and [22].) Multispectral image data collected by a highaltitude scanner were reduced to a form that could be viewed as images (pictures) of a shoreline in two dimensions. (See [23].) Data on several variables relating to yield and protein content were used to create an index to select parents of subsequent generations of improved bean plants. (See [13].) A matrix of tactic similarities was developed from aggregate data derived from professional mediators. From this matrix the number of dimensions by which professional mediators judge the tactics they use in resolving disputes was determined. (See [21].)
Sorting and grouping Data on several variables related to computer use were employed to create clusters of categories of computer jobs that allow a better determination of existing (or planned) computer utilization. (See [2].) Measurements of several physiological variables were used to develop a screening procedure that discriminates alcoholics from nonalcoholics. (See [26].) Data related to responses to visual stimuli were used to develop a rule for separating people suffering from a multiplesclerosiscaused visual pathology from those not suffering from the disease. (See Exercise 1.14.)
4
Chapter 1 Aspects of Multivariate Analysis
.~
T1.3 The Organization of Data
The Organization of Data 5
The U.S. Internal Revenue Service uses data collected from tax returns to sort taxpayers into two groups: those that will be audited and those that will not. (See [31].)Investigation of the dependence among variables
The preceding descriptions offer glimpses into the use of multivariate methods in widely diverse fields.
Data on several variables were used to identify factors that were responsible for client success in hiring external consultants. (See [12].) Measurements of variables related to innovation, on the one hand, and variables related to the business environment and business organization, on the other hand, were used to discover why some firms are product innovators and some firms are not. (See [3].) Measurements of pulp fiber characteristics and subsequent measurements of . characteristics of the paper made from them are used to examine the relations between pulp fiber properties and the resulting paper properties. The goal is to determine those fibers that lead to higher quality paper. (See [17].) The associations between measures of risktaking propensity and measures of socioeconomic characteristics for toplevel business executives were used to assess the relation between risktaking behavior and performance. (See [18].). Prediction
Throughout this text, we are going to be concerned with analyzing measurements made on several variables or characteristics. These measurements (commonly called data) must frequently be arranged and displayed in various ways. For example, graphs and tabular arrangements are important aids in data analysis. Summary numbers, which quantitatively portray certain features of the data, are also necessary to any description. We now introduce the preliminary concepts underlying these first steps of data organization.
ArraysMultivariate data arise whenever an investigator, seeking to understand a social or physical phenomenon, selects a number p ~ 1 of variables or characters to record . The values of these variables are all recorded for each distinct item, individual, or experimental unit. We will use the notation Xjk to indicate the particular value of the kth variable that is observed on the jth item, or trial. That is,Xjk =
The associations between test scores, and several high school performance variables, and several college performance variables were used to develop predictors of success in college. (See [10).) Data on several variables related to the size distribution of sediments were used to develop rules for predicting different depositional environments. (See [7] and [20].) Measurements on several accounting and financial variables were used to develop a method for identifying potentially insolvent propertyliability insurers. (See [28].) cDNA microarray experiments (gene expression data) are increasingly used to study the molecular variations among cancer tumors. A reliable classification of tumors is essential for successful diagnosis and treatment of cancer. (See [9].)Hypotheses testing
measurement ofthe kth variable on the jth item
Consequently, n measurements on p variables can be displayed as follows: Variable 1 Item 1: Item 2:Itemj: Itemn:Xu X21 Xjl Xnl
Variable 2X12 X22 Xj2 Xn2
VariablekXlk X2k Xjk Xnk
Variable pxl p X2p Xjp xnp
Several pollutionrelated variables were measured to determine whether levels for a large metropolitan area were roughly constant throughout the week, or whether there was a noticeable difference between weekdays and weekends. (See Exercise 1.6.) Experimental data on several variables were used to see whether the nature of the instructions makes any difference in perceived risks, as quantified by test scores. (See [27].) Data on many variables were used to investigate the differences in structure of American occupations to determine the support for one of two competing sociological theories. (See [16] and [25].) Data on several variables were used to determine whether different types of firms in newly industrialized countries exhibited different patterns of innovation. (See [15].)
Or we can display these data as a rectangular array, called X, of n rows and p columns:XllX21
X12 Xn
Xlk X2k
xl p X2p Xjp x np
XXjl Xj2 Xjk Xnl Xn2 Xnk
The array X, then, contains the data consisting of all of .the observations on all of the variables.
6
Chapter 1 Aspects of MuItivariate Analysis
Example 1.1 (A data array) A selection of four receipts from a university bookstore was obtained in order to investigate the nature of book sales. Each receipt provided, among other things, the number of books sold and the total amount of each sale. Let the first variable be total dollar sales and the second variable be number of books sold. Then we can re&ard the corresponding numbers on the receipts as four measurements on two variables. Suppose the data, in tabular form, are
r I
The Organization of Data
7
If the n measurements represent a subset of the full set of measurements that might have been observed, then Xl is also called the sample mean for the first variable. We adopt this terminology because the bulk of this book is devoted to procedUres designed to analyze samples of measurements from larger collections. The sample mean can be computed from the n measurements on each of the p variables, so that, in general, there will be p sample means:Xk
Variable 1 (dollar sales): 42 52 48 58 Variable 2 (number of books): 4 5 4 3 Using the notation just introduced, we haveXll = X12 =
= 
2: Xjk n j=l1~(j=l
1
n
k = 1,2, ... ,p
(11)
A measure of spread is provided by the sample variance, defined for n measurements on the first variable asX4l X42
42 4
X2l X22
= 52 = 5
X3l X32
= 48 = 4
= 58 = 3whereXl
2 SI
=  "'" Xjl 
n
_2 xd
and the data array X is X =
with four rows and two columns.
l
is the sample mean of the2 Sk
XiI'S.
In general, for p variables, we have)2.
42 52 48 58
4l 5 4 3
=  "'" Xjk  Xk
1 ~ (j=l
_
n
k = 1,2, ... ,p
(12)
Considering data in the form of arrays facilitates the exposition of the subject matter and allows numerical calculations to be performed in an orderly and efficient manner. The efficiency is twofold, as gains are attained in both (1) describing numerical calculations as operations on arrays and (2) the implementation of the calculations on computers, which now use many languages and statistical packages to perform array operations. We consider the manipulation of arrays of numbers in Chapter 2. At this point, we are concerned only with their value as devices for displaying data.
,
1\vo comments are in order. First, many authors define the sample variance with a divisor of n  1 rather than n. Later we shall see that there are theoretical reasons for doing this, and it is particularly appropriate if the number of measurements, n, is small. The two versions of the sample variance will always be differentiated by displaying the appropriate expression. Second, although the S2 notation is traditionally used to indicate the sample variance, we shall eventually consider an array of quantities in which the sample variances lie along the main diagonal. In this situation, it is convenient to use double subscripts on the variances in order to indicate their positions in the array. Therefore, we introduce the notation Skk to denote the same variance computed from measurements on the kth variable, and we have the notational identitiesk=I,2, ... ,p
Descriptive StatisticsA large data set is bulky, and its very mass poses a serious obstacle to any attempt to visually extract pertinent information. Much of the information contained in the data can be assessed by calculating certain summary numbers, known as descriptive statistics. For example, the arithmetic average, or sample mean, is a descriptive statistic that provides a measure of locationthat is, a "central value" for a set of numbers. And the average of the squares of the distances of all of the numbers from the mean provides a measure of the spread, or variation, in the numbers. We shall rely most heavily on descriptive statistics that measure location, variation, and linear association. The formal definitions of these quantities follow. Let Xll, X2I>"" Xnl be n measurements on the first variable. Then the arithmetic average of these measurements is
(13)
I I
The square root of the sample variance, ~, is known as the sample standard deviation. This measure of variation uses the same units as the observations. Consider n pairs of measurements on each of variables 1 and 2:
[xu],X12
[X2l], .. , [Xnl] X22 Xn2
That is, Xjl and Xj2 are observed on the jth experimental item (j = 1,2, ... , n). A measure of linear association between the measurements of variables 1 and 2 is provided by the sample covariance
8
Chapter 1 Aspects of Multivariate Analysis
fif
or the average product of the deviations from their respective means. If large values for one variable are observed in conjunction with large values for the other variable, and the small values also occur together, sl2 will be positive. If large values from one variable occur with small values for the other variable, Sl2 will be negative. If there is no particular association between the values for the two variables, Sl2 will be approximately zero. The sample covarianceSik
j
The Organization of Data, 9
= :Lnj=l
1
n
_
~
(Xji  Xi)(Xjk  Xk)
i
= 1,2, ... ,p,
k
=
1,2, ... ,p (14)
measures the association between the ith and kth variables. We note that the covariance reduces to the sample variance when i = k. Moreover, Sik = Ski for all i and k .. The final descriptive statistic considered here is the sample correlation coefficient (or Pearson's productmoment correlation coefficient, see [14]). This measure of the linear association between two variables does not depend on the units of measurement. The sample correlation coefficient for the ith and kth variables is defined as
The ~u~ntities Sik and rik do not, in general, convey all there is to know about the aSSOCIatIOn between two variables. Nonlinear associations can exist that are not revealed .by these ~es~riptive statistics. Covariance and corr'elation provide measures of lmear aSSOCIatIOn, or association along a line. Their values are less informative ~~r other kinds of association. On the other hand, these quantities can be very sensIttve to "wild" observations ("outIiers") and may indicate association when in fact, little exists. In spite of these shortcomings, covariance and correlation coefficien~s are routi':lel.y calculated and analyzed. They provide cogent numerical summan~s ~f aSSOCIatIOn ~hen the data do not exhibit obvious nonlinear patterns of aSSOCIation and when WIld observations are not present. . Suspect observa.tions must be accounted for by correcting obvious recording mIstakes and by takmg actions consistent with the identified causes. The values of Sik and rik should be quoted both with and without these observations. The sum of squares of the deviations from the mean and the sum of crossproduct deviations are often of interest themselves. These quantities areWkk
:L (Xji j=l
n
=
2: (Xjk j=I
n
Xk)2
k = 1,2, ... ,p
(16)
x;) (Xjk  Xk)(15)
andWik =
2: (Xji j=l
n
x;) (Xjk  Xk)
i
=
1,2, ... ,p,
k = 1,2, ... ,p
(17)
= 1,2, ... , p and k = 1,2, ... , p. Note rik = rki for all i and k. The sample correlation coefficient is a standardized version of the sample covariance, where the product of the square roots of the sample variances provides the standardization. Notice that rik has the same value whether n or n  1 is chosen as the common divisor for Sii, sa, and Sik' The sample correlation coefficient rik can also be viewed as a sample co variance. Suppose the original values 'Xji and Xjk are replaced by standardized valuesfor i(Xji  xi)/~and(xjk  xk)/~.Thestandardizedvaluesarecommensurablebe
The descriptive statistics computed from n measurements on p variables can also be organized into arrays.
Arrays of Basic Descriptive StatisticsSample means
cause both sets are centered at zero and expressed in standard deviation units. The sample correlation coefficient is just the sample covariance of the standardized observations. Although the signs of the sample correlation and the sample covariance are the same, the correlation is ordinarily easier to interpret because its magnitude is bounded. To summarize, the sample correlation r has the following properties:
i~mSn =
1. The value of r must be between 1 and +1 inclusive. 2. Here r measures the strength of the linear association. If r = 0, this implies a lack of linear association between the components. Otherwise, the sign of r indicates the direction of the association: r < 0 implies a tendency for one value in the pair to be larger than its average when the other is smaller than its average; and r > 0 implies a tendency for one value of the pair to be large when the other value is large and also for both values to be small together. 3. The value of rik remains unchanged if the measurements of the ith variable are changed to Yji = aXji + b, j = 1,2, ... , n, and the values of the kth variable are changed to Yjk = CXjk + d, j == 1,2, ... , n, provided that the constants a and c have the same sign.
Sample variances and covariances
[uS~lSpl 'pI
Sl2
S22
S2p
'"
sp2
spp
Sample correlations
R
~ l~'
r12
1'p2
'"
r2p
] ]
(18)
1
lE
10
Chapter 1 Aspects of Multivariate Analysis The sample mean array is denoted by X, the sample variance and covari~nce array by the capital letter Sn, and the sample correlation array by R. The subscrIpt ~ on the array Sn is a mnemonic device used to remind you that n is employed as a divisor for the elements Sik' The size of all of the arrays is determined by the number of variables, p. The arrays Sn and R consist of p rows and p columns. The array x is a single column with p rows. The first subscript on an entry in arrays Sn and R indicates the row; the second subscript indicates the column. Since Sik = Ski and rik = rki for all i and k, the entries in symmetric positions about the main northwestsoutheast diagonals in arrays Sn and R are the same, and the arrays are said to be symmetric.Example 1.2 (The arrays ;c, SR' and R for bivariate data) Consider the data intro
The Organization of Data The sample correlation isr12 r21
II
= ,=
vs;; VS;R _ [
Sl2
1.5
V34 v'3
. = .36
=
rl2
so
.36
1
.3~J
lE
Graphical Techniquesare im~ortant, but frequently neglected, aids in data analysis. Although it is impossIble to simultaneously plot all the measurements made on several variables and study ~he configurations, plots of individual variables and plots of pairs of variables can stIll be very informative. Sophisticated computer programs and display equipn;tent al.low on~ the luxury of visually examining data in one, two, or three dimenSIOns WIth relatIve ease. On the other hand, many valuable insights can be obtained from !he data by const~uctin~ plots with paper and pencil. Simple, yet elegant and ~ffectIve, met~ods for ~IsplaYIllg data are available in [29]. It is good statistical practIce to plot paIrs of varIables and visually inspect the pattern of association. Consider, then, the following seven pairs of measurements on two variables: Variable 1 Variable2 .Thes~
Plot~
duced in Example 1.1. Each. receipt yields a pair of measurements, total dollar sales, and number of books sold. Find the arrays X, Sn' and R. Since there are four receipts, we have a total of four measurements (observations) on each variable. Thesample means are4
Xl
= 1 2:j=l4
Xjl
= 1(42 +
52 + 48
+ 58) = 50
X2
= 12: Xj2 = ~(4 + 5 + 4 + 3) = 4j=l
(Xl):(X2):
3 5
4 5.5
2 4
6 7
8 10
2 5
5 7.5
The sample variances and covariances areSll =
~
4
data ~re ?lotted as seven points in two dimensions (each axis representIll~ a vanable) III FIgure 1.1. The coordinates of the points are determined by the patr~d measurements: (3,5), (4,5.5), ... , (5,7.5). The resulting twodimensional plot IS known as a scatter diagram or scatter plot.
2: j=l
(Xjl 
xd+ (58  50)2) = 34X2 X2
= ~42S22 =
 50)2 + (52  50l + (48  50)2(Xj2 
j=l ~ 14  4f + (5  4? + (4  4f + (3  4)2)4
~ 2:
4
10
10
xd
=
.5
! '" :a= 1.5
Sl2 = ~ 2: j=l
(Xjl 
XI)( Xj2

X2)
CS Cl
86 42
86 42
= ~42  50)(4  4)
+ (52  50)(5  4) + (48  50)(4  4) + (58  50)(3  4
S21 = Sl2 and 34 1.5J
0
4
6
8
!Sn = [ 1.5
2
!4
!
!
5
8 6 Dot diagram
10
I ..
XI
Figure 1.1 A scatter plot and marginal dot diagrams.
12 Chapter 1 Aspects of Multivariate Analysis Also shown in Figure 1.1 are separate plots of the observed values of variable 1 and the observed values of variable 2, respectively. These plots are called (marginal) dot diagrams. They can be obtained from the original observations or by projecting the points in the scatter diagram onto each coordinate axis. The information contained in the singlevariable dot diagrams can be used to calculate the sample means Xl and X2 and the sample variances SI 1 and S22' (See Exercise 1.1.) The scatter diagram indicates the orientation of the points, and their coordinates can be used to calculate the sample covariance s12' In the scatter diagram of Figure 1.1, large values of Xl occur with large values of X2 and small values of Xl with small values of X2' Hence, S12 will be positive. Dot diagrams and scatter plots contain different kinds of information. The information in the marginal dot diagrams is not sufficient for constructing the scatter plot. As an illustration, suppose the data preceding Figure 1.1 had been paired differently, so that the measurements on the variables Xl and X2 were as follows: Variable 1 Variable 2(Xl): (X2):
The Organization of Data
13
Example 1.3 (The effect of unusual observations on sample correlations) Some fi . nancial data representing jobs and productivity for the 16 largest publishing firms appeared in an article in Forbes magazine on April 30, 1990. The data for the pair of variables Xl = employees Gobs) and X2 = profits per employee (productivity) are graphed in Figure 1.3. We have labeled two "unusual" observations. Dun & Bradstreet is the largest firm in terms of number of employees, but is "typical" in terms of profits per employee. TIme Warner has a "typical" number of employees, but comparatively small (negative) profits per employee.X2
40
5
45.5
5
6 4
2 7
2 10
85
3 7.5
S,''~
8';,'
30
~::: 0~
0
(We have simply rearranged the values of variable 1.) The scatter and dot diagrams for the "new" data are shown in Figure 1.2. Comparing Figures 1.1 and 1.2, we find that the marginal dot diagrams are the same, but that the scatter diagrams are decidedly different. In Figure 1.2, large values of Xl are paired with small values of X2 and small values of Xl with large values of X2' Consequently, the descriptive statistics for the individual variables Xl, X2, SI 1> and S22 remain unchanged, but the sample covariance S12, which measures the association between pairs of variables, will now be negative. The different orientations of the data in Figures 1.1 and 1.2 are not discernible from the marginal dot diagrams alone. At the same time, the fact that the marginal dot diagrams are the same in the two cases is not immediately apparent from the scatter plots. The two types of graphical procedures complement one another; they are nqt competitors. The next two examples further illustrate the information that can be conveyed by a graphic display.
Co]
20
f
~
tE
~
100
,0
Time Warner
Dun & Bradstreet
1
10Employees (thousands)
Figure 1.3 Profits per employee and number of employees for 16 publishing firms.
IIf
The sample correlation coefficient computed from the values of Xl and X2 is .39 .56 = { _ .39 .50 for all 16 firms for all firms but Dun & Bradstreet for all firms but Time Warner for all firms but Dun & Bradstreet and Time Warner
r12
X2
X2
108
108
4
It is clear that atypical observations can have a considerable effect on the sample correlation coefficient.
6
6
42
4202
6
8
10
XI
t
t4
t6
t8
2
I 10
... XI
Figure 1.2 Scatter plot and dot diagrams for rearranged data.
Example 1.4 (A scatter plot for baseball data) In a July 17,1978, article on money in sports, Sports Illustrated magazine provided data on Xl = player payroll for National League East baseball teams. We have added data on X2 = wonlost percentage "for 1977. The results are given in Table 1.1. The scatter plot in Figure 1.4 supports the claim that a championship team can be bought. Of course, this causeeffect relationship cannot be substantiated, because the experiment did not include a random assignment of payrolls. Thus, statistics cannot answer the question: Could the Mets have won with $4 million to spend on player salaries?
14 Chapter 1 Aspects of Multivariate AnalysisTable 1.1 1977 Salary and Final Record for the National League EastX2=
The Organization of DataTable 1.2 PaperQuality Measurements
15
Team Philadelphia Phillies Pittsburgh Pirates St. Louis Cardinals Chicago Cubs Montreal Expos New York Mets
Xl =
player payroll 3,497,900 2,485,475 1,782,875 1,725,450 1,645,575 1,469,800
wonlost percentage .623 .593 .512 .500 .463 .395
Strength Specimen 1 2 3 4 5 6 7 8 9 10 Density .801.~24
Machine direction 121.41 127.70 129.20 131.80 135.10 131.50 126.70 115.10 130.80 124.60 118.31 114.20 120.30 115.70 117.51 109.81 109.10 115.10 118.31 112.60 116.20 118.00 131.00 125.70 126.10 125.80 125.50 127.80 130.50 127.90 123.90 124.10 120.80 107.40 120.70 121.91 122.31 110.60 103.51 110.71 113.80
Cross direction 70.42 72.47 78.20 74.89 71.21 78.39 69.02 73.10 79.28 76.48 70.25 72.88 68.23 68.12 71.62 53.10 50.85 51.68 50.60 53.51 56.53 70.70. 74.35 68.29 72.10 70.64 76.33 76.75 80.33 75.68 78.54 71.91 68.22 54.42 70.41 73.68 74.93 53.52 48.93 53.67 52.42
11
o
Player payroll in millions of dollars
Figure 1.4 Salaries and wonlost percentage from Table 1.1.
To construct the scatter plot in Figure 1.4, we have regarded the six paired observations in Table 1.1 as the coordinates of six points in twodimensional space. The figure allows us to examine visually the grouping of teams with respect to the variables total payroll and wonlost percentage. 
Example I.S (Multiple scatter plots for paper strength measurements) Paper is manufactured in continuous sheets several feet wide. Because of the orientation of fibers within the paper, it has a different strength when measured in the direction produced by the machine than when measured across, or at right angles to, the machine direction. Table 1.2 shows the measured values ofXlX2X3
= density (grams/cubic centimeter) = strength (pounds) in the machine direction= strength (pounds) in the cross direction
A novel graphic presentation of these data appears in Figure 1.5, page' 16. The scatter plots are arranged as the offdiagonal elements of a covariance array and box plots as the diagonal elements. The latter are on a different scale with this
12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41
.841 .816 .840 .842 .820 .802 .828 .819 .826 .802 .810 .802 .832 .796 .759 .770 .759 .772 .806 .803 .845 .822 .971 .816 .836 .815 .822 .822 .843 .824 .788 .782 .795 .805 .836 .788 .772 .776 .758
Source: Data courtesy of SONOCO Products Company.
=The Organization of Data 16 Chapter 1 Aspects of Multivariate Analysis DensityMax0.97 Strength (MD) Strength (CD)
17
n Points in p Dimensions (pDimensional Scatter Plot). Consider the natural extension of the scatter plot to p dimensions, where the p measurements
i "0
Med Min
~....
0.81 0.76
..Max
.. . ... .. .. .e' . . :~.:
:
.. ;135.1
.::..:.:. '.. . .... ..
~ S" ~ '"
.. ...... ... ...
OIl
...r
..
Med
r r
T'
I
I
121.4
... ..Max
: :
.. .... ... ... .'.
on the jth item represent the coordinates of a point in pdimensional space. The coordinate axes are taken to correspond to the variables, so that the jth point is Xjl units along the first axis, Xj2 units along the second, ... , Xjp units along the pth axis. The resulting plot with n points not only will exhibit the overall pattern of variability, but also will show similarities (and differences) among the n items. Groupings of items will manifest themselves in this representation. The next example illustrates a threedimensional scatter plot.
..
Min
103.5
.. 4*.:.*~:\.
*'
.... .. .. ......:. .. ... .
...
T
80.33
Med
70.70
. :....
Min
48.93
Example 1.6 (Looking for lowerdimensional structure) A zoologist obtained measurements on n = 25 lizards known scientifically as Cophosaurus texanus. The weight, or mass, is given in grams while the snoutvent length (SVL) and hind limb span (HLS) are given in millimeters. The data are displayed in Table 1.3. Although there are three size measurements, we can ask whether or not most of the variation is primarily restricted to two dimensions or even to one dimension. To help answer questions regarding reduced dimensionality, we construct the threedimensional scatter plot in Figure 1.6. Clearly most of the variation is scatter about a onedimensional straight line. Knowing the position on a line along the major axes of the cloud of poinfs would be almost as good as knowing the three measurements Mass, SVL, and HLS. However, this kind of analysis can be misleading if one variable has a much larger variance than the others. Consequently, we first calculate the standardized values, Zjk = (Xjk  Xk)/~' so the variables contribute equally to the variation
Figure 1.5 Scatter plots and boxplots of paperquality data from Thble 1.2. software so we use only the overall shape to provide information on symme~ry and possible outliers for each individual characteristic. The scatter plots can be mspected for patterns and unusual observations. In Figure 1.5, there is one unusual observation: the density of specimen 25. Some of the scatter plots have patterns suggesting that there are two separate clumps of observations. These scatter plot arrays are further pursued in our discussion of new software graphics in the next section. Table 1.3 Lizard Size Data Lizard 1 2 3 4 5 6 7 8 9 10 11 12 13 Mass 5.526 10.401 9.213 8.953 7.063 6.610 11.273 2.447 15.493 . 9.004 8.199 6.601 7.622 SVL 59.0 75.0 69.0 67.5 62.0 62.0 74.0 47.0 86.5 69.0 70.5 64.5 67.5 HLS 113.5 142.0 124.0 125.0 129.5 123.0 140.0 97.0 162.0 126.5 136.0 116.0 135.0 Lizard 14 15 16 17 18 19 20 21 22 23 24 25 Mass 10.067 10.091 10.888 7.610 7.733 12.015 10.049 5.149 9.158 12.132 6.978 6.890 SVL 73.0 73.0 77.0 61.5 66.5 79.5 74.0 59.5 68.0 75.0 66.5 63.0 HLS 136.5 135.5 139.0 118.0 133.5 150.0 137.0 116.0 123.0 141.0 117.0 117.0
In the general multiresponse situation, p variables are simultaneously rec~rded oon items. Scatter plots should be made for pairs of important variables and, If the . task is not too great to warrant the effort, for all pairs. . Limited as we are to a three:dimensional world, we cannot always picture an entire set of data. However, two further geom7tri~ repres~nta~ions of t?e. data provide an important conceptual framework for Vlewmg multIvanable statlstlc~l methods. In cases where it is possible to capture the essence of the data m three dimensions, these representations can actually be graphed.
Source: Data courtesy of Kevin E. Bonine.
Data Displays and Pictorial Representations 1 AspectS 0IS f Multivariate Analysis
19
Cbapter
Figure 1.8 repeats the scatter plot for the original variables but with males marked by solid circles and females by open circles. Clearly, males are typically larger than females.
15~ ~
10 5
.......80 90
15
\oTl~
155 135 5060
o ~.Figure 1.6 3D scatter plot of lizard data from Table 1.3.
X2, ... , xp) of P can vary to produce different locations for the point. The data that determine distance will, however, remain fixed. To illustrate, suppose we have n pairs of measurements on two variables each having mean zero. Call the variables Xl and X2, and assume that the Xl measurements vary independently of the X2 measurements, I In addition, assume that the variability in the X I measurements is larger than the variability in the X2 measurements. A scatter plot of the data would look something like the one pictured in Figure 1.20.X2
Cherooff faces have also been used to display differences in m~ltivariate ob~er vations in two dimensions. For example, the twodi~ensional coordInate ~xes ffilght resent latitude and longitude (geographical locatiOn), and the faces mIght repr~ ::~t multivariate measurements on several U.S. cities. Additional examples of thiS kind are discussed in [30]. .... . There are several ingenious ways to picture multIvanate data m two dimensiOns. We have described some of them. Further advance~ are possible and will almost certainly take advantage of improved computer graphICs.
I::'
1.5 Distance they may at first appear formida?le, ~ost multiv~ate tec~niques are based Although
upon the simple concept of distance. StraIght~e, or Euclidean, d~stan~e sh~uld be familiar. If we consider the point P 0= (Xl ,.X2) III th~ plane, the straIghtlIne dIstance, d(O, P), from P to the origin 0 = (0,0) IS, accordmg to the Pythagorean theorem,
d(O,p)=Vxi+x~
(19)
The situation is illustrated in Figure 1.19. In general, if the point P has p coo:d.inates so that P = (x), X2, .. ' x p ), the straightline distance from P to the ongm0= (O,O, ... ,O)is
d(O,P)
0=
Vxr + x~ + ... + x~
(110)
(See Chapter 2.) All points (Xl> X2, ... : xp) thatlie a constant squared distance, such as c2, from the origin satisfy the equatIon 2 d2(O, P) = XI + x~ + ... + x~ = c (111) Because this is the equation of a hypersphere (a circle if p = 2), points equidistant from the origin lie on a hypersphere. .. . . The straightline distance between two arbItra~y ~OInts P and Q WIth COordInatesP = (XI,X2, ... ,X p ) andQ 0= (Yl>Y2,,Yp)lsglVenbyd(P,Q) = V(XI  YI)2 + (X2  )'z)2 + ... + (xp  Yp)2(112)
Figure 1.20 A scatter plot with greater variability in the Xl direction than in the X2 direction.
Straightline, or Euclidean, distance is unsatisfactory for most stat~stical purp~s es. This is because each coordinate contributes equally to the calculatlOn of ~uchd ean distance. When the coordinates r~prese~t .measurem~nts that ar~ subject t~ andom fluctuations of differing magmtudes, It IS often deslfable to weIght CO?rdl ~ates subject to a great deal of variability less ~eavily than those that are not highly variable. This suggests a different measure ?f ~lst,~n~e. . Our purpose now is to develop a "staUstlcal distance that ac:counts for dIfferences in variation and, in due course, the presence of correlatlOn. Because our
Glancing at Figure 1.20, we see that values which are a given deviation from the origin in the Xl direction are not as "surprising" or "unusual" as ~re values equidistant from the origin in the X2 direction. This is because the inherent variability in the Xl direction is greater than the variability in the X2 direction. Consequently, large Xl coordinates (in absolute value) are not as unexpected as large X2 coordinates. It seems reasonable, then, to weight an X2 coordinate more heavily than an Xl coordinate of the same value when computing the "distance" to the origin. . One way to proceed is to divide each coordinate by the sample standard deviatIOn. Therefore, upon division by the standard deviations, we have the "standardized" coordinates x; = xIi";;;; and x; = xz/vS;. The standardized coordinates are now on an equal footing with one another. After taking the differences in variability into account, we determine distance using the standard EucIidean formula. Thus, a statistical distance of the point P = (Xl, X2) from the origin 0 = (0,0) can be computed from its standardized coordinates x~ = xIiVS;; and xi 0= X2/VS; asd(O, P) =V(xD2= )(
+ (x;)2
~y + (
Js;y
(113)
=
Figure 1.19 Distance given by the Pythagorean theorem.
IAt this point, "independently" means that the Xz measurements cannot be predicted with any accuracy from the Xl measurements, and vice versa.
32
Chapter 1 Aspects of Multivariate Analysis Comparing (113) with (19), we see that the difference between the two expressions is due to the weights kl = l/s11 and k2 = l/s22 attached to xi and x~ in (1l3). Note that if the sample variances are the same, kl = k 2 , then xI and x~ will receive the same weight. In cases where the weights are the same, it is convenient to ignore the common divisor and use the usual Euc1idean distance formula. In other words, if the variability in thexl direction is the same as the variability in the X2 direction, and the Xl values vary independently of the X2 values, Euc1idean distance is appropriate. Using (113), we see that all points which have coordinates (Xl> X2) and are a constant squared distance c2 from the origin must satisfy
Distance 33
Coordinates: (Xl, X2)
. XI x~ DIstance'.  + 4 10212
=
1
(0,1) (0,1) (2,0) (1, \/3/2)
+= 1 4 1
0 2 (1)2 +=1
4
1
(114) .Equation (114) is the equation of an ellipse centered at the origin whose major and minor axes coincide with the coordinate axes. That is, the statistical distance in (113) has an ellipse as the locus of all points a constant distance from the origin. This general case is shown in Figure 1.21.
4" +
12
22 02 + =1 4 1 (\/3/2)21= 1
. A pl?t ?f the equation xt/4 + xVI = 1 is an ellipse centered at (0,0) whose major. aXIS he~ along the Xl coordinate axis and whose minor axis lies along the X2 coordmate aXIS. The halflengths of these major and minor axes are v'4 = 2 and VI = 1, :espectively. The ellipse of unit distance is plotted in Figure 1.22. All points on the ellIpse are regarded as being the same statistical distance from the originin this case, a distance of 1. x,
__~~4~r_~~~X,
cJs;:
_z::rJ'jL..+*x,I Z
Figure 1.21 The ellipse of constant
Figure 1.22 Ellipse of unit
statistical distance d 2(O,P) = xI!sll + X~/S22 = c 2.
I
distance, 4 +
.
xi
x~ 1
=
1.
Example 1.14 (Calculating a statistical distance) A set of paired measurements (Xl, X2) on two variables yields Xl = X2 = 0, Sll = 4, and S22 = 1. Suppose the Xl
measurements are unrelated to the x2 measurements; that is, measurements within a pair vary independently of one another. Since the sample variances are unequal, we measure the square of the distance of an arbitrary point P = (Xl, X2) to the origin 0= (0,0) by
The expression in (113) can be generalized to accommodate the calculation of statistical distance from an arbitrary point P = (Xl, X2) to any fIXed point Q = (YI, )'z). ~f we assume that .the coordinate variables vary independently of one another, the dIstance from P to Q is given by d(P, Q) =
\.j
I
(Xl Sl1
YI)2
+
(X2 S22
)'z)2
'(115)
All points (Xl, X2) that are a constant distance 1 from the origin satisfy the equation.!.+2= 1x2 x2
4
1
.The extension of this statistical distance to more than two dimensions is straIghtforward. Let the points P and Q have p coordinates such that P = (x~, X2,, xp) and Q = (Yl,)'z, ... , Yp). Suppose Q is a fixed point [it may be the ongm 0 = (0,0, ... , O)J and the coordinate variables vary independently of one another. Let Su, s22,"" spp be sample variances constructed from n measurements on Xl, X2,"" xp, respectively. Then the statistical distance from P to Q is d(P,Q) =
The coordinates of some points a unit distance from the origin are presented in the following table:
~(XI sll
Yl? + (X2  )'z)2 + ... + (xp  Yp)2 s22 spp
(116)
34 C
bapter 1
Aspects of Multivar iate Analysis
Distance
35
All points P that are a constan t squared distance from Q rle on a hyperellipsoid d' t d at Q whose major and minor axes are parallel to the coor ma e ax es. We centere . note th~ followmg: 1. The distance of P to the origin 0 is obtained by setting Yl = )'2 = ... = YP=
The relation between the original coordin ates (Xl' Xz) and the rotated coordinates (Xl, X2) is provide d by
Xl = Xl cos (0) + x2sin(0 ) X2= Xl sin (8) + X2 cos (8)(118)
0
in (116). 
Z If Sll
_
_ .,. =spp'

S22 
. . the Euclidean distance formula m (112 ).IS appropna te.
The distance in (116) still does not include most of the i~porta~cases ~erSphl~! f the assumption of indepen dent coordmates. e sca e enc~unteri ~;c::;~~ a twodimensional situation in which the xl ~easur~m~nts ~o io FIgure. . f h X measurements. In fact, the coordmates 0 t e p~Irs o.ot vary mdepen dently 0 t e 2 mall together and the sample correlatIOn ) h'b't a tendenc y to b e 1 arge or s ' (.~lf~~ie~ i~ ;ositive . Moreov er, the variability in the X2 direction is larger than t h e coe . d' f variability.m the Xl . Ifgfec ::~asure of distance when the variability in the Xl direcWhat IS a meamn u . . h variability in the X2 directio n an d t h e vana bles X 1 and X 2 tion is dl~~r~~t :~~a:lyewe can use what we have already intro~uced, provided t~at are corre a e . . . '. wa From Fi ure 1.23, we see that If we rotate the ong;,e ihe angle: while keeping the scatter fixed and lOa) cO ~ d the scatter in terms of the new axes looks very ~uc . the r?tat~d axe; ou 2~ay wish to turn the book to place the Xl and X2 a.xes m tha~ 10 FIgure . ~sitions.) This suggests that we calculate the .sample vananc~ s theIr cust~mar~ coordin ates and measure distance as in EquatIOn (113). That.Is, using the Xl an 2 h ~ d X axes we define the distance from the pomt 'th reference to t e Xl an 2 ' ; =' (Xl, X2) to the origin 0 = (0,0) as
Given the relation s in (118), we can formally substitu te for Xl and X2 in (117) and express the distance in terms of the original coordinates. After some straight forward algebraic manipul ations, the distance from P = (Xl, X2) to the origin 0 = (0,0) can be written in terms of the original coordinates Xl and X2 of Pas d(O,P) = Val1x1 + 2al2xlx2 + a22x~ (119) where the a's are number s such that the distance is nonnega tive for all possible values of Xl and X2. Here all, a12, and a22 are dete,rmined by the angle 8, and Sll, s12, and S22 calculat ed from the original data. 2 The particul ar forms for all, a12, and a22 are not importa nt at this point. What is importa nt is the appeara nce of the crossproduct term 2a12xlxZ necessit ated by the nonzero correlat ion r12' Equatio n (119) can be compar ed with (113). The expressi on in (113) can be regarde d as a special case of (119) with all = 1/s , a22 = 1/s , and a12 = O. ll 22 In general, the statistic al distance ofthe point P = (x], X2) from the fvced point Q = (Yl,)'2) for situatio ns in which the variable s are correlat ed has the general form)'2) + azz(x2 )'2? (120) and can always be comput ed once all, a12, and a22 are known. In addition , the coordinates of all points P = (Xl, X2) that are a constan t squared distance 2 c from Q satisfy
lOO~:;i:I~g:;~:!: :~;~~gh x
;0 c;.f
~~~:~
d(P,Q) = Val1(X I 
yd + 2adxI
 YI)(XZ 
d(O, P) =
(117)
denote the sample variances comput ed with the Xl arid X2 where Sl1 and sn measurements.X2
~
Xl ~
.,
.,:~
1
(121) By definition, this is the equatio n of an ellipse centere d at Q. The graph of such an equatio n is displayed in Figure 1.24. The major (long) and minor (short) axes are indicated. They are parallel to the Xl and 1'2 axes. For the choice of all, a12, and a22 in footnote 2, the Xl and X2 axes are at an angle () with respect to the Xl and X2 axes. The general ization of the distance formula s of (119) and (120) to p dimensions is straight forward . Let P = (Xl,X2 ,""X ) be a point whose coordin ates p represe nt variable s that are correlat ed and subject to inheren t variability. Let2Specifically,sin2(6) all = coS1(O)SIl + 2sin(6)co s(/I)SI2 + sin2(O)s12 + cos2(8)S22  2sin(8)oo s(8)sl2 + sin2(8}slI 2 sin2(/I} oos (8) a22 = cos2(8}SII + 2 sin(lI}cOS(8}SI2 + sin2(6)S22 + cos2(9)sn  2sin(8)oos (/I}SI2 + sin2(8)sll cos2(8)
al1(xl 
yd 2 +
2adxI  YI)(X2 
)'2)
+ a22(x2 
)'2)2 =
c2
.
8
__~~~~~~Xl II
. ,..I.
1
Figure 1.23 A scatter plot for positively correlated measurements and a rotated coordinate system.
and
cos(lI) sin(/I} sin(6} oos(/I} al2 = cos2(II)SIl + 2 sin(8) cos(8)sl2 + sin2(8)~2  cog2(/J)S22  2 sin(/J} ooS(6)812 + sin2(/I}sll
36
Chapter 1 Aspects of Multivariate AnalysisX2
Exercises 37
/
[email protected] :.. 
: .....: . ..
..... .. .... .... .. .. ... ..oFigure 1.25 A cluster of points relative to a point P and the origin.
/
XI
//
""
"
"
Figure 1.24 Ellipse of points a constant distance from the point Q.
od(O,P) =
0) denote the origin, and let Q = (YI, Y2, ... , Yp) be a speC!"fd le fix;d p~i~i.Then the distances from P to 0 and from Pto Q have the general (0 0forms.~
________________~________~______~__~~~ allx1 + a22x~ + ... + appx~ + 2a12xlx2 + 2a13Xlx3 + ... + 2a p_l,px p_IX p (122)
and [aJ1(xI d(P,Q)
yd + a22(x2 +
Y2)2 + .. , + app(xp Yp)2 + 2an(xI YI)(X 2__ Y2) 2a13(XI  YI)(X3  Y:l) + ... + 2apl,p(xp1  YpI)(X p Yp)] (123). 3
The need to consider statistical rather than Euclidean distance is illustrated heuristically in Figure 1.25. Figure 1.25 depicts a cluster of points whose center of gravity (sample mean) is indicated by the point Q. Consider the Euclidean distances from the point Q to the point P and the origin O. The Euclidean distance from Q to P is larger than the Euclidean distance from Q to O. However, P appears to be more like the points in the cluster than does the origin. If we take into account the variability of the points in the cluster and measure distance by the statistical distance in (120), then Q will be closer to P than to O. This result seems reasonable, given the nature of the scatter. Other measures of distance can be advanced. (See Exercise 1.12.) At times, it is useful to consider distances that are not related to circles or ellipses. Any distance measure d(P, Q) between two points P and Q is valid provided that it satisfies the following properties, where R is any other intermediate point:d(P, Q) = d(Q, P) d(P,Q) > OifP Q d(P,Q) = OifP = Q d(P,Q) :5 d(P,R) + d(R,Q)
*
where the a's are numbers such that the distances are always nonnegatIve. . We note that the distances in (122) and (123) are completely dete~~llned by .). . the coeffiCIents (weIghts aik> I  1, 2 , ... , p, k.  1,'2 , ... , P. These coeffIcIents can be set out in the rectangular array
(125) (triangle inequality)
r::: ::~ :::]la]p a2p a: p
1.6 Final CommentsWe have attempted to motivate the study of multivariate analysis and to provide you with some rudimentary, but important, methods for organizing, summarizing, and displaying data. In addition, a general concept of distance has been introduced that will be used repeatedly in later chapters.
(124)
the a 's with i k are displayed twice, since they are multiplied by 2 in the h were ,k . . h' 'fy the distance func distance formulas. Consequently, the entnes m t IS array specI . The a. 's cannot be arbitrary numbers; they must be such that the computed t1OnS. ,k . f . (S E . 110 ) distance is nonnegative for every paIr 0 pomts. ee xerclse . . Contours of constant distances computed from (122) a~d. \123) .are ereIlipsoids. A hypereIIipsoid resembles a football when p = 3; It IS Impossible hY P . . . to visualize in more than three dlmens~ons.lJbe 81 ebraic expressions for the squares of the distances in ,,1 + a,,1>,1 + a"b,J.. ]b',Example 2.5 (Matrix multiplication) If
~ [7
3 6) [
!J ~
113)
The product b' c is a 1A= [ 1
X
1 vector or a single number, here 13.
3 1 2J
54'
then 3 A B = [ (2X3)(3Xl) 1 1 2J [2] = [3(2) + (1)(7) + 2(9)J 5 4 ~ 1( 2) + 5(7) + 4(9)
bc' =
[
3 [5 8 4] =6
7]
[35 5615 24 30 48
28] 12 24
The product b c' is a matrix whose row dimension equals the dimension of band whose column dimension equals that of c. This product is unlike b' c, which is a single number.
and
(2~2)(2~3)

G~J[~ ! !J+ 0(1) 1(3)  1(1)2 4J 6 2(2x3)
= [2(3)
2(1) + 0(5) 2(2) + 0(4)J 1(1)  1(5) 1(2)  1(4)
The product d' A b is a 1
X
1 vector or a single number, here 26.
=
[~
Square matrices will be of special importance in our development of statistical methods. A square matrix is said to be symmetric if A = A' or aij = aji for all i andj.
58 Chapter 2 Matrix Algebra and Random Vectors so
Some Basics of Matrix and Vector Algebra 59
Example 2.1 (A symmetric matrix) The matrix
[is symmetric; the matrix is AI. We note that
.2 .8
.4J .6
is not symmetric.
implies that Cl = C2 = 0, so the columns of A are linearly independent. This confirms the condition stated in (212). A method for computing an inverse, when one exists, is given in Supplement 2A. The routine, but lengthy, calculations are usually relegated to a computer, especially when the dimension is greater than three. Even so, you must be forewarned that if the column sum in (212) is nearly 0 for some constants Cl, .. , Ck, then the computer may produce incorrect inverses due to extreme errors in rounding. It is always good to check the products AAI and AI A for equality with I when AI is produced by a computer package. (See Exercise 2.10.) Diagonal matrices have inverses that are easy to compute. For example,
When two square matrices A and B are of the same dimension, both products AB and BA are defined, although they need not be equal. (See Supplement 2A.) If we let I denote the square matrix with ones on the diagonal and zeros elsewhere, it follows from the definition of matrix multiplication that the (i, j)th entry of AI is ail X 0 + ... + ai.jI X 0 + aij X 1 + ai.j+1 X 0 + .. , + aik X 0 = aij, so AI = A. Similarly, lA = A, so(kXk)(kxk)
I
A
=
(kxk)(kXk)
A
I
=
(kXk)
A
for any A
(kxk)
(211)
The matrix I acts like 1 in ordinary multiplication (1 a = a '1= a), so it is called the identity matrix. The fundamental scalar relation about the existence of an inverse number aI such that ala = aaI = 1 if a =f. 0 has the following matrix algebra extension: If there exists a matrix B such that(kXk)(kXk)
1all
0
o o1
o o o1
o o o o1
0a22
BA=AB=I(kXk)(kXk)
(kXk)
0 0a33
then B is called the inverse of A and is denoted by AI. The technical condition that an inverse exists is that the k columns aI, a2, ... , ak of A are linearly indeperident. That is, the existence of AI is equivalent to (212) (See Result 2A.9 in Supplement 2A.)Example 2.8 (The existence of a matrix inverse) For
[1
0 0 0
0 0 0a44
0 0
~ 1h~mvm'a55
0 0 0
1a22
0 0
0
o o
o
o
o
if all the aH =f. O. Another special class of square matrices with which we shall become familiar are the orthogonal matrices, characterized by
A=[! you may verify that [ .2 .8
~J(.2)2 (.8)2
QQ' = Q'Q
=I
or
Q'
= QI
(213)
.6
.4J [3 4
2J = 1
[(.2)3 + (.4)4 (.8)3 + (.6)4[~ ~J
+ (.4)1 + (.6)1
J
The name derives from the property that if Q has ith row qi, then QQ' = I implies that qiqi ;: 1 and qiqj = 0 for i =f. j, so the rows have unit length and are mutually perpendicular (orthogonal).According to the condition Q'Q = I, the columns have the same property. We conclude our brief introduction to the elements of matrix algebra by introducing a concept fundamental to multivariate statistical analysis. A square matrix A is said to have an eigenvalue A, with corresponding eigenvector x =f. 0, if
=
Ax
=
AX
(214)
,p60 Chapter 2 Matrix Algebra and Random Vectors Positive Definite Matrices 61
Ordinarily, we normalize x so that it has length unity; that is, 1 = x'x. It is convenient to denote normalized eigenvectors bye, and we do so in what follows. Sparing you the details of the derivation (see [1 D, we state the following basic result: Let A be a k X k square symmetric matrix. Then A has k pairs of eigenvalues and eigenvectorsnamely, (215) The eigenvectors can be chosen to satisfy 1 = e; el = ... = e"ek and be mutually perpendicular. The eigenvectors are unique unless two or more eigenvalues are equal.
multivariate analysis. In this section, we consider quadratic forms that are always nonnegative and the associated positive definite matrices. Results involving quadratic forms and symmetric matrices are, in many cases, a direct consequence of an expansion for symmetric matrices known as the spectral decomposition. The spectral decomposition of a k X k symmetric matrix A is given by1 A(kXk)
= Al e1
e;
(kX1)(lxk)
+ ..1.2 e2 ez + ... + Ak ek eA:(kX1)(lXk)
(216)
(kx1)(lXk)
Example 2.9 (Verifying eigenvalues and eigenvectors) Let
where AI, A , ... , Ak are the eigenvalues of A and el, e2, ... , ek are the associated 2 normalized eigenvectors. (See also Result 2A.14 in Supplement 2A). Thus, eiei = 1 for i = 1,2, ... , k, and e:ej = 0 for i j.
*
A 
[1 5J5.
Example 2.1 0 (The spectral decomposition of a matrix) Consider the symmetric matrix
1
Then, since
A =
[
4 2
13 4 2]13 2 2 10
Al = 6 is an eigenvalue, and
The eigenvalues obtained from the characteristic equation I A  AI I = 0 are Al = 9, A2 = 9, and ..1.3 = 18 (Definition 2A.30). The corresponding eigenvectors el, e2, and e3 are the (normalized) solutions of the equations Aei = Aiei for i = 1,2,3. Thus, Ael = Ae1 gives
or is its corresponding normalized eigenvector. You may wish to show that a second eigenvalueeigenvector pair is ..1.2 = 4, = [1/v'2,I/\I2].
ez
13ell  4ell
4e21 2e21
+
2e31 = gel1
+
13e21 
2el1 
+ 10e31
2e31 = ge21 = ge31
A method for calculating the A's and e's is described in Supplement 2A. It is instructive to do a few sample calculations to understand the technique. We usually rely on a computer when the dimension of the square matrix is greater than two or three.
2.3 Positive Definite MatricesThe study of the variation and interrelationships in multivariate data is often based upon distances and the assumption that the data are multivariate normally distributed. Squared distances (see Chapter 1) and the multivariate normal density can be expressed in terms of matrix products called quadratic forms (see Chapter 4). Consequently, it should not be surprising that quadratic forms play a central role in
Moving the terms on the right of the equals sign to the left yields three homogeneous equations in three unknowns, but two of the equations are redundant. Selecting one of the equations and arbitrarily setting el1 = 1 and e21 = 1, we find that e31 = O. Consequently, the normalized eigenvector is e; = [1/VI2 + 12 + 02, I/VI2 + 12 + 02, 0/V12 + 12 + 02] = [1/\12, 1/\12,0], since the sum of the squares of its elements is unity. You may verify that ez = [1/v18, 1/v'I8, 4/v'I8] is also an eigenvector for 9 = A , and e3 = [2/3, 2/3, 1/3] is the normalized eigenvector corresponding 2 to the eigenvalue A3 = 18. Moreover, e:ej = 0 for i j.
*
lA proof of Equation (216) is beyond the scope ofthis book. The interested reader will find a proof in [6), Chapter 8.
62
Chapter 2 Matrix Algebra and Random Vectors
Positive Definite Matrices 63
The spectral decomposition of A is thenA = Alelel
Example 2.11 (A positive definite matrix and quadratic form) Show that the matrix
or
[
13 13 4 4 2 2
2 2 10
J
= 9
[~l_1_
Vi
[
+ Azezez + A3 e 3e 3
for the following quadratic form is positive definite:3xI
+ 2x~  2Vi XlxZ
1 Vi
To illustrate the general approach, we first write the quadratic form in matrix notation as(XI XZ{
o1
vJ V;] [;J= Aiel ej(2XIJ(IXZ)
= x/Ax
VIS+91
VIS4
[~
4 ] VIS vT8 + 18
1
VIS1 18 1 18 4 18 1 18 1 18 4 184 ~
2 3 2 3 1 3
[~
By Definition 2A.30, the eigenvalues of A are the solutions of the equation  AI I = 0, or (3  A)(2  A)  2 = O. The solutions are Al = 4 and Az = l. Using the spectral decomposition in (216), we can write
IA
A(ZXZ)
+
Azez
ei
(ZXIJ(JXZ)
= 4el e;(ZXI)(IX2)
+ e2 ei(ZXIJ(IXZ)
18 4 18 16 18
where el and e2 are the normalized and orthogonal eigenvectors associated with the eigenvalues Al = 4 and Az = 1, respectively. Because 4 and 1 are scalars, premuItiplication and postmultiplication of A by x/ and x, respectively, where x/ = (XI' xz] is any non zero vector, givex/ A x=
4x'
el
ej
x
+
x/
ez
ei
x
(I XZ)(2xZ)(ZXI)
(I XZ)(ZXI)(I X2)(ZX 1)
(IXZ)(2XI)(1 X2)(ZXI)
= 4YI
+ y~;:,: 0and Yz
+
4 9 4 18 9 2 9
4 9 4 9 2 9
2 9 2 9 1 9
with YI
= x/el
= ejx
= x/ez
= eix
We now show that YI and Yz are not both zero and, consequently, that x/ Ax = 4YI + y~ > 0, or A is positive definite. From the definitions of Y1 and Yz, we have
as you may readily verify.
ory(ZXI)
The spectral decomposition is an important analytical tool. With it, we are very easily able to demonstrate certain statistical results. The first of these is a matrix explanation of distance, which we now develop. Because x/ Ax has only squared terms xt and product terms XiXb it is caIled a quadratic form. When a k X k symmetric matrix A is such that (217) Os x/A x for all x/ = (XI' Xz, ... , xd, both the matrix A and the quadratic form are said to be nonnegative definite. If equality holds in (217) only for the vector x/ = (0,0, ... ,0], then A or the quadratic form is said to be positive definite. In other words, A is positive definite if (218) 0< x/Ax for all vectors x~
=
E X (ZX2)(ZXI)
Now E is an orthogonal matrix and hence has inverse E/. Thus, x = E/y. But x is a nonzero vector, and 0 ~ x = E/y implies that y ~ O. Using the spectral decomposition, we can easily show that a k X k symmetric matrix A is a positive definite matrix if and only if every eigenvalue of A is positive. (See Exercise 2.17.) A is a nonnegative definite matrix if and only if all of its eigenvalues are greater than or equal to zero. Assume for the moment that the p elements XI, Xz, ... , Xp of a vector x are realizations of p random variables XI, Xz, ... , Xp. As we pointed out in Chapter 1,
O.
Chapter 2 Matrix Algebra and Random Vectors
A S