Stratified sampling

A probability sampling technique that is distinguished by the two-step procedure where:
1) the parent population is divided into mutually exclusive and exhaustive subsets or strata, and
2) a simple random sample of elements is chosen independently from each strata or subset. The strata should be mutually exclusive and collectively exhaustive in that every population element should be assigned to one and only one stratum and no population elements should be omitted. There are two variants of stratified random sampling: proportionate and disproportionate sampling.