stratified sample

A stratified sample is a survey in which not all survey units are sampled with equal probability.

The American Community Survey is an example of a stratified sample because certain minority and geographic groups are sampled with higher probability.

In a stratified sample, each surveyed individual represents a different number of people in the population. For example, if 1 in 1,000 white people are surveyed, then each white person in the data represents 1,000 people in the population. But if 1 in 500 black people are surveyed, then each black person in the data represents 500 people in the population. The number of people that a survey unit represents is called the population weight.

When computing summary statistics in stratified samples, it is important to take into account the population weights.