Stratified random sampling (usually referred to simply as stratified sampling) is a type of probability sampling that allows researchers to improve precision (reduce error) relative to simple random sampling (SRS). The population is divided into non-overlapping groups, or strata, along a relevant dimension such as gender, ethnicity, political affiliation, and so on. The researcher then collects a random sample of population members from within each stratum. This technique ensures that observations from all relevant strata are included in the sample.