All people in sampling frame are divided into "strata" (groups or categories). Within each stratum, a simple random sample or systematic sample is selected.
Example of stratified sampling - If we want to ensure that a sample of 5 students from a group of 50 contains both male and female students in same proportions as in the full population (i.e. the group of 50), we first divide that population into male and female. In this case, there are 22 male students and 28 females. To work out the number of males and females in the sample........
No. of females in sample = (5 / 50) x 28 = 2.8
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