A frequency polygon is used to display grouped data
To draw a frequency polygon we plot the midpoint of the group against the frequency and then join the points with straight lines
Here is a frequency table showing the time, in minutes, it took for 25 students to get to school one day
| Time | Frequency |
|---|---|
| 0 ≤ t ≤ 10 | 7 |
| 10 ≤ t ≤ 20 | 9 |
| 20 ≤ t ≤ 30 | 6 |
| 30 ≤ t ≤ 40 | 3 |
To draw a frequency polygon we plot the midpoint with each group against the frequency
The midpoint of 0 and 10 is 5
We could work this out by adding 0 and 10, and then dividing by 2: (0 + 10)⁄2 = 5
For the first point we plot (5, 7)
The midpoint of 10 and 20 is 15
(10 + 20)⁄2 = 15
The second point is (15,9)
The midpoint of 20 and 30 is 25
(20 + 30)⁄2 = 25
The next point is (25,6)
The midpoint of 30 and 40 is 35
(30 + 40)⁄2 = 35
The next point is (35,3)
And finally we join the points with straight lines
