Stratum in Stratified Sampling
In stratified sampling, strata are non-overlapping groups within the population, each defined by a characteristic (age, gender, income level). Random samples are drawn independently from each stratum in proportions reflecting population composition.
Real World
The UK government's Annual Population Survey divides the population into strata by region and age group, then randomly samples within each stratum to ensure rural communities and the elderly are not under-represented.
Exam Focus
When defining strata, state the characteristic used and confirm the groups are non-overlapping and exhaustive.
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