probability sampling

a form of sampling in which each case or element in the population has a known probability of being included in the sample. In the simplest situation of simple random sampling each case has the same probability of being sampled. In stratified random sampling each case may not have the same probability of being included as the population is divided into different strata or groups which may not have the same probability of being chosen. Probability sampling may be distinguished from non-probability sampling in which the probability of a case being selected is unknown. Non-probability sampling is often euphemistically described as convenience sampling.