Frequency Polygons
by David M. Lane
Prerequisites
• Chapter 2: Histograms
Learning Objectives
- Create and interpret frequency polygons
- Create and interpret cumulative frequency polygons
- Create and interpret overlaid frequency polygons
Frequency polygons are a graphical device for understanding the shapes of distributions. They serve the same purpose as histograms, but are especially helpful for comparing sets of data. Frequency polygons are also a good choice for displaying cumulative frequency distributions.
To create a frequency polygon, start just as for histograms, by choosing a class interval. Then draw an X-axis representing the values of the scores in your data. Mark the middle of each class interval with a tick mark, and label it with the middle value represented by the class. Draw the Y-axis to indicate the frequency of each class. Place a point in the middle of each class interval at the height corresponding to its frequency. Finally, connect the points. You should include one class interval below the lowest value in your data and one above the highest value. The graph will then touch the X-axis on both sides.
A frequency polygon for 642 psychology test scores shown in Figure 1 was constructed from the frequency table shown in Table 1.
Table 1. Frequency Distribution of Psychology Test Scores
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