Definition:Stratified Random Sample

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Definition

Let $P$ be a population.

Let $S \subsetneq P$ be a sample.


Then $S$ is a stratified random sample if and only if:

there exists a finite expansion $P = P_1 \mid P_2 \mid \cdots \mid P_k$ of $P$ such that $S$ is the union of $k$ simple random samples, one taken from each $P_i$.


Strata

The components of $P$ are called strata, singular stratum.