Classifying and interpreting data

• Classify data: categorical or numerical; numerical data is discrete (counted) or continuous (measured).
• Tabulate data in tally tables, frequency tables and two-way tables.
• Read, interpret and draw inferences from tables and statistical diagrams.
• Compare data sets using an average and a measure of spread, in context.
• Know the limits: small or biased samples, outliers and missing information restrict the conclusions you can draw.

Categories, counts and measurements

Continuous data is grouped with inequalities so that every value fits exactly one class, for example 150 < h ≤ 160. Discrete data can be grouped as 1–5, 6–10 and so on.

Counting in fives

A tally table records data as you go through it, one stroke per item, with every fifth stroke crossing the previous four. The frequency is the total of each tally. Work through the list once, in order, rather than counting each value separately.

Two ways of sorting at once

A two-way table sorts the same people by two things at once, such as gender and favourite sport. Each row and each column has a total, and the grand total sits in the corner. To complete one, look for a row or column with only one gap, fill it, and repeat.

An average and a spread, in context

To compare two data sets, make two comparisons: one using an average (mean, median or mode) and one using a measure of spread (range or interquartile range). Each comparison must say which group is higher and what that means in the context of the question.