A frequency polygon is a way of picturing data by plotting the class mark on the horizontal axis and the frequency of the class on the vertical axis and connecting the points. The polygon is completed by extending the class marks one-class width on either end with a frequency of zero for both.
A frequency polygon can be drawn according to the following steps.
- Draw a histogram from the given data.
- Join the consecutive mid-points of the upper horizontal sides of the rectangles in the histogram.
- It is assumed that the class preceding the first class and the class succeeding the last class in the classification exists and the frequency of each of them is zero. Class marks of these classes are joined with the mid-points of the upper horizontal sides of the extreme rectangles of the histogram.
The figure above shows the frequency polygon drawn with the help of the histogram. Observe it carefully. A frequency polygon can be drawn without drawing a histogram.
Applying the method of point plotting, a frequency polygon can be drawn as follows:
