Where the population embraces many distinct categories, the frame can be organized by these categories into separate "strata. " Each stratum is then sampled as an independent sub-population, out of which individual elements can be randomly selected. In this way, researchers can draw inferences about specific subgroups that may be lost in a more generalized random sample. Additionally, since each stratum is treated as an independent population, different sampling approaches can be applied to different strata, potentially enabling researchers to use the approach best suited for each identified subgroup. Stratified sampling can increase the cost and complicate the research design.
Source: Boundless. “Sampling Techniques.” Boundless Political Science. Boundless, 26 May. 2016. Retrieved 08 Nov. 2016 from https://www.boundless.com/political-science/textbooks/boundless-political-science-textbook/public-opinion-6/measuring-public-opinion-46/sampling-techniques-272-1483/