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In social science research, snowball sampling is a technique for developing a research sample where existing study subjects recruit future subjects from among their acquaintances. Thus the sample group appears to grow like a rolling snowball. As the sample builds up you gain enough data to use for your data. This sampling technique is often used in hidden populations which are difficult for researchers to access; example populations would be drug users or commercial prostitutes.

Because sample members are not selected from a sampling frame, snowball samples are subject to numerous biases. For example, people who have many friends are more likely to be recruited into the sample.

It was widely believed that it was impossible to make unbiased estimates from snowball samples, but a variation of snowball sampling called respondent-driven sampling has been shown to allow researchers to make asymptotically unbiased estimates from snowball samples under certain conditions. Respondent-driven sampling also allows researchers to make estimates about the social network connecting the hidden population.