A public opinion poll in Ohio wants to determine whether registered voters in the state approve of a measure to ban smoking in all public areas. The researchers select a simple random sample of 50 registered voters from each county in the state and ask whether they approve or disapprove of the measure. This is an example of A. a systematic county sample. B. a stratified sample. C. a multistage sample. D. a simple random sample.

A group-based sampling process that can be divided into subpopulations. For statistical studies, testing of each subpopulation separately may be useful if subpopulations within a total population differ, thus understood as "Stratified sampling".

One might, for instance, divide a adults sample into subgroups in terms of age, like 18 to 29, 30 to 39, 40 to 49, 50–59 etc with decided age difference as needed. A stratified sample may be more accurate than an easy sample of the similar size by random. As it offers more accuracy, a stratified sample sometimes involves a smaller sample, saving money.