Learn in the textbookPopulation and Sampling
DefinitionStratified sampling (層別抽出) divides the population into groups of similar elements (strata) and chooses a sample at random from each stratum.
ExplanationStrata are formed by traits likely to affect the results, such as age or region. If each stratum's sample size is set in proportion to its size, the sample mirrors the population's makeup. Simple random sampling can by chance pick almost no one from some stratum, but dividing into strata first keeps that kind of bias in check.
AnalogyIt is like checking the quality of a fruit gift set by taking a few pieces from each of the apple, orange, and grape boxes.
Memory hookGeologists call each layer of rock a "stratum." Picture taking soil from every layer in a cliff face to test it.