**Probability-proportional-to-size sampling** is a type of multistage cluster sampling. In this method, the probability of selecting an element in any given cluster varies inversely with the size of the cluster.

Although cluster sampling and stratified sampling bear some superficial similarities, they are substantially different. In stratified sampling, a random sample is drawn from all the strata, where in cluster sampling only the selected clusters are studied, either in single stage or multi stage.

*See also : statistics, marketing research, quantitative marketing research, sampling, cluster sampling, simple random sampling, nonprobability sampling, systematic sampling, stratified sampling*