Stratified Sampling
A sampling method that divides the population into non-overlapping subgroups (strata) based on shared characteristics, then random samples are drawn from each stratum, typically proportional to stratum size.
Real World
NHS researchers studying patient satisfaction stratify hospitals by size (small, medium, large) and sample proportionally from each group, ensuring smaller hospitals are not swamped by large ones in the results.
Exam Focus
Calculate the number sampled from each stratum using (stratum size ÷ population size) × sample size, and show each step.
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