"In stratified random sampling, the population is first divided into two or more strata. The strata can be based on a single criterion (e.g., sex, yielding the two strata of male and female) or on a combination of two or more criteria (e.g., age and sex, yielding strata, such as females under 21, females 21 and over, males under 21, and males 21 and over). In stratified random sampling, a simple random sample is taken from each stratum, and the subsamples are then joined to form the total sample."