Stratified sample - GCSE Maths Definition

A stratified sample is a random sampling technique that has the added restriction that we would only require a certain number of observations/responses in each stratum. A stratum (the plural of which is strata) is a group with a defining characteristic or property - for example, a stratum could be hair colour, or age.

Stratified samples are designed so that the proportion of observations in each stratum within the sample, is (roughly) equal to the proportion of that stratum within the population. For example, if the population has 40% of people with brown hair, and we took a sample size of 200, we would require 40% of 200 = 80 people in the same to have brown hair. Taking a stratified sample helps ensure the sample is representative of the population.