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Statistics for the Social Sciences. Psychology 340 Spring 2005. Distributions. Outline (for week). Variables: IV, DV, scales of measurement Discuss each variable and it’s scale of measurement Characteristics of Distributions Using graphs Using numbers (center and variability) - PowerPoint PPT Presentation
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Statistics for the Social SciencesPsychology 340
Spring 2005
Distributions
Statistics for the Social Sciences
Outline (for week)
• Variables: IV, DV, scales of measurement– Discuss each variable and it’s scale of measurement
• Characteristics of Distributions– Using graphs– Using numbers (center and variability)
• Descriptive statistics decision tree
• Locating scores: z-scores and other transformations
Statistics for the Social Sciences
Let’s get some distributions
• On a sheet of paper (that you’ll turn in) write out these pieces of information:– Male or female– Height (in inches)– How many pairs of shoes in your closet– Typical number of servings of soda per day– Typical number of servings of water per day
• Okay, turn these in
Statistics for the Social Sciences
Basic Concepts
Variable– A condition or characteristic that can have different
values Value
– A possible number or category that a score can have Score
– A particular person’s value on a variable
Statistics for the Social Sciences
Basic Concepts
Kinds of Variables– A condition or characteristic that can have different
values– Experiment:
– Independent - manipulated by experimenter– Dependent - measured by experimenter
– Observational:– Explanatory - observed variable to do the explaining– Response - variable to be predicted
Statistics for the Social Sciences
Measurement
• Properties of our measurement?– Units of measurement - whether the measurement has a
minimum sized unit or not– Levels (Scales) of measurement - the correspondence
between the numbers representing the properties that we’re measuring
Statistics for the Social Sciences
Units of Measurement
• Continuous variables– Variables can take any number and can be infinitely
broken down into smaller and smaller units– E.g., For lunch I can have 2, 3, or 2.5 cookies
• Discrete variables– Broken into a finite number of discrete categories that can’t be broken down– E.g., In my family I can have
1 kidor 2 kids, but not 2.5
Statistics for the Social Sciences
Levels (scales) of measurement
• Nominal Scale: Consists of a set of categories that have different names. – Measurements on a nominal scale label and categorize
observations, but do not make any quantitative distinctions between observations.
– Example:• Eye color:
blue, green, brown, hazel
Statistics for the Social Sciences
Levels of measurement
• Ordinal Scale: Consists of a set of categories that are organized in an ordered sequence. – Measurements on an ordinal scale rank observations in
terms of size or magnitude.– Example:
• T-shirt size: Small, Med, Lrg, XL, XXL
Statistics for the Social Sciences
Levels of measurement
• Interval Scale: Consists of ordered categories where all of the categories are intervals of exactly the same size. – With an interval scale, equal differences between
numbers on the scale reflect equal differences in magnitude.
– Ratios of magnitudes are not meaningful.– Example:
• Fahrenheit temperature scale20º40º
“Not Twice as hot”
Statistics for the Social Sciences
Levels of measurement
• Ratio scale: An interval scale with the additional feature of an absolute zero point. – With a ratio scale, ratios of numbers DO reflect ratios of
magnitude.
Statistics for the Social Sciences
Our variables
• Descrete or continuous?• What level of measurement?
– Male or female– Height (in inches)– How many pairs of shoes in your closet– Typical number of servings of soda per day– Typical number of servings of water per day
• Now let’s consider how we would describe one of these variables
Statistics for the Social Sciences
Distributions
• The distribution of a variable is a description of all of the tokens of the variable within in sample (or population if you’ve got the data)– A picture of the distribution is usually helpful
• Gives a good sense of the properties of the distribution
– Many different ways to display distribution• Frequency distribution table• Graphs
Statistics for the Social Sciences
Steps for Making a Frequency Table(do this for class soda drinking variable)
• Make a list down the page of each possible value, from highest to lowest
X f % c% 12 11 10 9 8 7 6 5 4 3 2
The values of the variable
Statistics for the Social Sciences
Steps for Making a Frequency Table
• Go one by one through the scores, making a mark for each next to its value on the list, count up how frequently each value appears and include this in the table
X f % c% 12 11 10 9 8 7 6 5 4 3 2
The values of the variable
The number of tokens of each variable
Statistics for the Social Sciences
Steps for Making a Frequency Table
• Figure the percentage (or proportion) of scores for each value
X f % c% 12 11 10 9 8 7 6 5 4 3 2
The values of the variable
The number of tokens of each variable
N=total% = (f/N)*100
The percentage of tokens at each value
Statistics for the Social Sciences
Steps for Making a Frequency Table
• Figure the cumulative percentage (or proportion) of scores for each value
X f % c% 12 11 10 9 8 7 6 5 4 3 2
The values of the variable
The number of tokens of each variable
N=total% = (f/N)*100
The percentage of tokens at each value
Cumulative percentage
Statistics for the Social Sciences
Grouped Frequency Table(do this for class height variable)
A frequency table that uses intervals (range of values) instead of single values
Statistics for the Social Sciences
Frequency Graphs
Histogram Plot the
different values against the frequency of each value
Statistics for the Social Sciences
Frequency Graphs
Histogram (create one for class height) Step 1: make a frequency distribution table
(may use grouped frequency tables) Step 2: put the values along the bottom, left to
right, lowest to highest Step 3: make a scale of frequencies along left
edge Step 4: make a bar above each value with a
height for the frequency of that value
Statistics for the Social Sciences
Frequency Graphs
Frequency polygon - essentially the same, put uses lines instead of bars
Statistics for the Social Sciences
Properties of distributions
• Distributions are typically summarized with three features
• Shape• Center• Variability (Spread)
Statistics for the Social Sciences
Shapes of Frequency Distributions
Unimodal, bimodal, and rectangular
Statistics for the Social Sciences
Shapes of Frequency Distributions
Symmetrical and skewed distributions
Normal and kurtotic distributions
Statistics for the Social Sciences
Displaying two variables
Bar graphs Can be used in a number of ways (including
displaying one or more variables) Best used for categorical variables
Scatterplots Best used for continuous variables
Statistics for the Social Sciences
Bar graphs
• Plot a bar graph of men and women in the class
• Plot a bar graph of shoes in closet crossed with men and women– What should we plot? (and why?)
• Total number of shoes for each group?• Average number of shoes for each group?
Statistics for the Social Sciences
Scatterplot
• Plot a scatterplot of soda and bottled water drinking– Useful for seeing the relationship between the
variables
Statistics for the Social Sciences
Next time
• In addition to using tables and graphs to describe distributions, we also can provide numerical summaries