PSYC 241 Lecture 2: Lesson 2: Frequency Distribution

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University of North Dakota
PSYC 241
Kristen Leighton

find more resources at 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 find more resources at find more resources at ○ 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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