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Stratum  (STRATUM)

In stratified random sampling1 the population is divided into a number of strata2 which are in some sense more homogeneous than the population as a whole with respect to the characteristics studied, and a simple random sample (cf. 161-4) is drawn in each stratum. Variable sampling fractions (161-5) may be used in different strata. Multi-stage sampling3 is a method where the selection of the sample is carried out in several stages. A sample of primary sampling units (160-4) is first selected and each of these units is then regarded as a population from which a sub-sample4 is selected, and the process may be repeated. Where there is no good sampling frame, a sample of areas delimited on a map may be selected: this procedure is called area sampling5.