Concept of Measurement

• The ability to demonstrate change or relationship and to communicate those changes to others.

• Describes the quality or quantity of an existing variable e.g. ROM and strength

Concept of Measurement

• Evaluate a patients condition and response to treatment. We measure change

• Is the process of assigning numerals to objects to represent quantitative characteristics according to certain rules.

Variable:

• a characteristic that can be manipulated or observed and that can take on different values, either– Quantitatively– Qualitatively

• The ability to measure a variable is dependent on one’s ability to define it.

• Continuous Variable: can take on any value along a continuum within a defined range

˚ (e.g. ROM 50.5 0 or 50 0)

˚ Discrete Variable: described only in whole numbers (e.g. HR)

˚ Construct: an abstract concept that is invented to represent unmeasureable behaviors or ideas

Rules of Measurement

• Nominal• Ordinal• Interval• Ratio

Rules of Measurement

Nominal - numerals represent category labels only, classifications

• Sex

• Blood Type

• Clinical Diagnosis

• Ordinal - numbers indicate rank order of observations

• MMT

• Pain

• Functional Status

Interval - equal intervals between numbers but not related to true zero, therefore not representing an absolute quantity

• IQ

• F0

• Calendar Years

Ratio - numbers represent units with equal intervals, measured from true zero

• Weight

• Strength

• BP

Reliability

• Reliability - the extent to which a measurement is consistent and free from error ( repeatability)

• Usefulness of measurement in clinical research and decision making depends on the extent to which the therapist can rely on data as accurate and meaningful indicator of behavior or attribute

Reliability

• Usefulness of measurement in clinical research and decision making

• Depends on the extent to which the therapist can rely on data as accurate and meaningful indicator of behavior or attribute

Measurement Errors

• Measurements are rarely perfectly reliable

• Error in Measurement

X = T + E

Observed Score True Score Error

Errors can be-

Systematic Errors: are predictable errors of measurement

Random Errors: due to chance (unpredictable)

Source of Measurement Error

• Rater error

• Inaccuracies in the measuring instruments

• variability of the characteristics being measured

Source of Measurement Error

Development of testing instruments involve a specific protocol that maximizes the reliability of the instrument. Errors are identified and then controlled or eliminated:

• Careful Planning

• Clear Operational Definitions

• Inspection of Equipment

Estimate of Reliability

True Score VarianceTrue Score Variance and Error

VarianceT

T + E

T + E T reliability increases

1.00 perfect reliability.00 no reliability

Correlation: degree of association between two sets of data or variables.

• Correlation between height and weight

• Not cause and effect research • State two variables are related (X,Y) • No true variable manipulation• Acceptable levels of positive and

negative correlations • A level of significance1.0 perfect.75 good.50 poor

Test - retest reliability

Establishes that an instrument is capable of measuring a variable with consistency. Analysis called test-rest reliability coefficient.

Effected by: 1 Testing Effects - practice or carry

over effect test2 Rater Bias - same rater can be

influenced by the memory of the first score. Controlled by blinding tester.

3-Test-Retest Interval time

4-Carryover & testing effects

Rater reliability - Rater is part of measurement system, in some cases is the actual instrument.

Intrarater Reliability - refers to the stability of data recorded by one individual across two or more trials

Interrater Reliability - concerns variation between two or more raters who measure the same group of subjects

Validity of Measurement

• Validity - concerns the extent to which an instrument measures what is is intended to measure.

• Implies that measurement is relatively free from error. Valid test must also be reliable.

• Low reliability low validity• High reliability is not automatically

valid• Validity helps us make inferences about

variables on relevant observable behavior or responses. Inferences go beyond the simple value assigned to them.

Specificity of Validity

• Face Validity - instrument appears to test what it is supposed to

• Content Validity - indicates that the items that make up an instrument adequately sample the universe of content that defines that variable being measured

Specificity of Validity

• Criterion-related validity - indicates the outcomes of one instrument, the target test that can be used as a substitute measure for an established gold standard criterion test, can be concurrent or predictive

Specificity of Validity

Concurrent validity - establishes validity when two measures are taken at relatively the same time.

Predictive validity - establishes that the outcome of the target test can be used to predict a future criterion score.

Prescriptive validity-establishes that the interpretation of a measurement is appropriate for determining effective intervention

Construct validity - establishes the ability of an instrument to measure an abstract construct and the degree to which the instrument reflects the theoretical components of the construct.

Specificity of Validity

Evaluating Diagnostic Procedures

A diagnostic test is used to screen for the presence or absence of a disease or abnormal condition

– Dichotomous

– Categorical

– Continuous

Evaluating Diagnostic Procedures

Sensitivity-the ability of the test to obtain a positive test when the target condition is present

Evaluating Diagnostic Procedures

Specificity-Is the ability of the test to obtain a negative test when the condition is really absent

Measuring Change

• Goal of tx is to effect a positive change

• Difference between outcome and initial is change or gain score

• Used to analyze the effect of tx or intervention

Goals

• Measure change in an individual performance or condition

• Measure differences between individuals in the amount of change

• Seek to identify factors that contribute to a good response

• Intend to draw inferences about tx effects by looking at group differences

Validity of change scores

• Level of measurement

• Reliability

• Stability

• Linearity