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School

University of North Dakota
Department

Psychology

Course Code

PSYC 241

Professor

Kristen Leighton

Lesson 2: Frequency Distribution

● We can describe any data set with three pieces of information

○ Shape of the distribution

○ Central tendency

○ Variability

● Distributions tell us:

○ Every possible score

○ Actual scores

● Frequency distribution

○ Show how many people are located in each category on the scale of measurement

○ Present an organized picture of the entire set of scores

○ Show where each person is located relative to others

● Frequency Distribution Tables

○ X column

■ Categories on the scale measurement

■ Values are listed from the highest to lowest (without skipping any)

○ Frequency column

■ Tallies for each value (how often each X value occurs in the data set)

■ Frequencies for each X value

■ The sum of the frequencies should equal N

○ The 'x' column contains every possible score

○ The 'f' column indicates the frequency of each score

● Grouped Frequency Distribution

○ When a frequency distribution table lists all of the individual categories (X values) it is

called a regular frequency distribution

○ This works well if we have very few scores

○ For many values (like exam scores), a grouped frequency distribution is used

○ The X column lists groups of scores, called class intervals, rather than individual values

○ Guidelines

■ ~10 class intervals

■ Width should be simple number (2, 5, 10, 20, etc.)

■ Bottom score should be a multiple of the width

■ All intervals should be the width with no gaps or overlaps

● Frequency distribution graphs

○ One alternative to tables is to create visual representations of the data

○ This allows us to more easily see the shape of a distribution

○ The score categories (X value) are listed on the X axis

○ The frequencies are listed on the Y axis

○ Scores from interval scale --> histogram or polygon

● Histograms

○ Bar is centered above each score (or class interval)

○ Height of the bar corresponds to the frequency

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