One type of random sampling employed in survey research is the use of disproportionate allocation to strata. Disproportionate allocation to strata sampling involves dividing the population of interest into mutually exclusive and exhaustive strata and selecting elements (e.g. households or persons) from each stratum.
Commonly used strata include geographic units; for example, high-minority-density census tracts in a city are put into one stratum and low-minority-density census tracts are put into another stratum. In epidemiology case-control studies, strata are used where persons in one stratum have a condition of interest (e.g. Type I diabetes) and persons without the condition are put into a second stratum. After dividing the population into two or more strata, a "disproportionate" number of persons are selected from one stratum relative to others. ...
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