The purpose of a frequency polygon is to communicate the shape of distributions. The purpose is similar to that of a histogram but is preferable when it comes to providing data comparisons. Those needing to display cumulative frequency distributions also find frequency polygrams to be more helpful.
Making a frequency polygon starts much the same way as with histograms. Selecting a class interval should be followed by drawing an X-axis that represents score values in the data and then using a small tick mark to indicate the middle of each interval. Then, the Y-axis indicates the frequency with which each class appears. A point in the middle of each interval goes at the height marking its frequency. Connecting the points yields the frequency polygon.
Cumulative frequency polygons have a similar purpose, but the Y value at each point shows the number in a particular class as well as the number in the intervals below. Drawing multiple frequency polygons on the same set of axes allows for a visual comparison that is much easier to understand than looking at two tables with data. This comparative tool provides statistical researchers with ways to make their findings simpler for the average reader to understand.