Frequency Polygons

Frequency polygons are obtained marking midpoints of the bars used in histogram and then the mid points joined using straight line.

Frequency polygon can also be obtained by plotting the midpoints of the classes against the frequencies.

Table of Contents

The table below shows the masses in kilograms of some 30 pigs measured by a certain farmer.

a table showing grouped masses of pigs

We are going to use the table above to draw a frequency polygon

we need to identify the mid-points of the classes because frequency should be plotted against the midpoint of the class.

The mid pints of each class is obtained by adding the boundaries together and then dividing by two.

The table below shows the class and their mid-points.

frequency table with class midpoints

The resulting polygon is as shown in the figure below

The length of 28 tree seedlings were recorded as follows in centimeters,

15 16 28 19 12 12 20 20 14 16 11 12 37 13 11 12 14 17 17 18 11 10 15 13 14 25 17 14 16 18.

(a) Make a grouped frequency table with equal widths of 4, starting from 11-14.

(b) on the same axis:

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