In statistics,<unk>  is a method of<unk>  from a population.
When sub-populations vary considerably, it is advantageous to sample each subpopulation (stratum) independently. Stratification is the process of grouping members of the population into relatively homogeneous subgroups before<unk> . The strata should be mutually exclusive: every element in the population  must be assigned to only one stratum. The strata should also be collectively exhaustive: no population element can be excluded. Then random or systematic<unk>  is applied within each stratum. This often improves the representativeness of the sample by reducing<unk>  error. It can produce a weighted mean that has less variability than the arithmetic mean of a simple random sample of the population.
Stratified sampling
Surrogate endpoint
Block design
Evidence-based design
Agent study
Placebo-controlled studies
Unblinding
Randomized controlled trial