Frequency table or grouped data

A standard method from Measures of central tendency: the steps and worked examples.

Steps

Mean = Σ(x × f) ÷ Σf, where x is the value (or class mark for grouped data).
Class mark = (lower limit + upper limit) ÷ 2.
Median: find the class containing the ((Σf + 1)/2)th item using cumulative frequency, or read it from a cumulative frequency curve.

Example 1: Grouped data.

Score	0–9	10–19	20–29	30–39
Class mark	4.5	14.5	24.5	34.5
Frequency	4	7	6	3

mean ≈ (4.5×4 + 14.5×7 + 24.5×6 + 34.5×3) ÷ 20
     = 370 ÷ 20
     = 18.5

For the whole topic, read Measures of central tendency: key points, common mistakes and how the DSE tests it.