This procedure, which may be used in conjunction with simple random, systematic, or cluster sampling, improves the representativeness of a sample, at least in terms of the stratification variables.

• 抽样前，将研究母群依据某分层的标准区分成同质的组别，如依据性别、年级将母体先分成同性别与同年级的组别。(母体愈同质，抽样误差愈小