Draw a frequency polygon for the following grouped frequency distribution table.
| Age of the donor (Yrs.) | 20–24 | 25–29 | 30–34 | 35–39 | 40–44 | 45–49 |
|---|---|---|---|---|---|---|
| No. of blood doners | 38 | 46 | 35 | 24 | 15 | 12 |
Answer: Plot (22, 38), (27, 46), (32, 35), (37, 24), (42, 15), (47, 12) and join them, closing at (17, 0) and (52, 0)
The classes are not continuous; making them continuous (19.5–24.5, 24.5–29.5, ...) does not change the class marks.
| Class | Class mark | Frequency | Point |
|---|---|---|---|
| 19.5–24.5 | 22 | 38 | (22, 38) |
| 24.5–29.5 | 27 | 46 | (27, 46) |
| 29.5–34.5 | 32 | 35 | (32, 35) |
| 34.5–39.5 | 37 | 24 | (37, 24) |
| 39.5–44.5 | 42 | 15 | (42, 15) |
| 44.5–49.5 | 47 | 12 | (47, 12) |
Take class marks (age of donor) on the X-axis and number of donors on the Y-axis with a convenient scale. Plot the points and also the points (17, 0) and (52, 0), which are the class marks of the imaginary classes just before the first and just after the last class (frequency 0). Join all the points in order by line segments:
(17, 0), (22, 38), (27, 46), (32, 35), (37, 24), (42, 15), (47, 12), (52, 0)
The closed figure obtained with the X-axis is the required frequency polygon.
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