Strata in stratified sampling are distinct subsets of all entries on a sampling frame. These subsets are strategically defined by one or more stratification variables to improve the statistical quality of findings for the population as a whole or for important population subgroups. Once strata are formed and the sample is allocated among strata, stratified sampling is accomplished by randomly selecting a sample within each stratum.

Sampling strata may be formed explicitly from a list of population members or as a part of the selection process in individual stages of a cluster sample. Strata may also be considered “implicit” when the frame is sorted by variables that could otherwise define strata, and sampling is done in each of a set of groups of neighboring entries throughout ...