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PSYC 241
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Kristen Leighton
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Lecture 2

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University of North Dakota

Psychology

PSYC 241

Kristen Leighton

Spring

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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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○ Width extends to the real limits (adjacent bars touch)
● Polygons
○ Dot is centered above each score
○ Height of the dot corresponds to the frequency
○ The dots are connected by straight lines
○ Additional line is drawn at each end to bring graph back to a zero frequency
● Bar graph

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