Read a cumulative frequency curve

A standard method from Geometry and data basics: the steps and worked examples.

Steps

The median is at half the total frequency.
Lower quartile at ¼, upper quartile at ¾ (used in F6).
Draw a horizontal line from the frequency axis to the curve, then down to read the value.

Worked examples

Example 1: Read a cumulative frequency curve: the curve for 80 students' marks passes through (52, 20), (61, 40) and (70, 60). Find the median and the interquartile range.

median: ½ × 80 = 40th student  → 61 marks
Q1:     ¼ × 80 = 20th student  → 52 marks
Q3:     ¾ × 80 = 60th student  → 70 marks
interquartile range = 70 − 52 = 18 marks

For the whole topic, read Geometry and data basics: key points, common mistakes and how the DSE tests it.

Steps

Worked examples

Other methods in this topic