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.
Example
The table below shows the masses in kilograms of some 30 pigs measured by a certain farmer.
| Class | Frequency |
| 25-29 | 6 |
| 30-34 | 12 |
| 35-39 | 8 |
| 40-44 | 5 |
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.
| class | Mid-point | Frequency |
| 25-29 | 27 | 6 |
| 30-34 | 32 | 12 |
| 35-39 | 37 | 4 |
| 40-44 | 42 | 5 |
The resulting polygon is as shown in the figure below
Example Question
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:
- draw a histogram to represent the data
- draw a frequency polygon
Related Topics
- Frequency tables
- bar graphs
- Introduction to statistics
- Measures of dispersion
- Grouped and ungrouped data
- bar graphs
- Line graphs
- Frequency polygon
- Histograms
- pie charts
- Measures of Central tendency
- Mean for grouped data
- Working with the assumed mean
- Quartiles, Deciles and percentiles
- Measures of dispersion
- Variance
- Mathematics