Sampling Theory of Surveys
Topic:
A simple random sample (SRS) is the simplest form of probability sample. An SRS of size n is taken when every possible subset of n units in the population has the same chance of being the sample.
In a stratified random sample, the population is divided into subgroups called strata. Then an SRS is selected from each stratum, and the SRSs in the strata are selected independently.