What is important to know about Frequency polygons?

Plot the midpoint of each class on the x-axis.

What is important to know about Frequency polygons?

Plot the frequency on the y-axis.

What is important to know about Frequency polygons?

Join the points with straight line segments.

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Statistics & Probability · Foundation & Higher

Frequency polygons

A frequency polygon is a line graph showing frequencies of grouped data. Plot the frequency against the midpoint of each class interval and join the points with straight lines.

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Key facts to remember

1Plot the midpoint of each class on the x-axis.
2Plot the frequency on the y-axis.
3Join the points with straight line segments.
4Do NOT join to the x-axis at either end.
5Used for comparing two distributions on the same axes.

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Worked examples

Example 1

Draw a frequency polygon for: 0–10 → 4; 10–20 → 9; 20–30 → 15; 30–40 → 7.

Working

Midpoints: 5, 15, 25, 35.
Plot (5, 4), (15, 9), (25, 15), (35, 7).
Join with straight line segments.

AnswerA polygon peaking at midpoint 25 with frequency 15.

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Common mistakes

✗Plotting at the class boundary instead of the midpoint.

✗Joining the first or last point down to the x-axis.

✗Confusing a frequency polygon with a histogram.

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Exam tips

✓Always use the midpoint of the interval, not the upper/lower bound.

✓Label both axes clearly.

✓When comparing two sets, draw both polygons on the same axes with a key.

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