Sample selection is said to be stratified if some form of random sampling is separately applied in each of a set of distinct groups formed from all of the entries on the sampling frame from which the sample is to be drawn. By strategically forming these groups, called "strata," stratification becomes a feature of sample designs that can improve the statistical quality of survey estimates. Procedurally, stratum formation and sample allocation to strata are important preliminary steps to sample selection. After the sample has been selected, data are gathered from its members, and analysis accommodating the use of stratification is conducted.

Stratification and sampling clusters are sometimes confused, as both involve groups of one form or another. These two design features are distinguishable by how ...

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