- This is a method of sampling in which existing study subjects recruit future subjects from among their acquaintances. Hence the recruits grow in snowball fashion. The technique is often used with hidden populations of people, such as drug users or prostitutes, who are otherwise difficult for researchers to access. The results can be less subject to selection bias than one might suppose. It is similar to respondent-driven sampling. However, whereas in snowball sampling the names of the contacts are handed over to the researcher and they initiate the invitation, in respondent-driven sampling, the participant does not hand over the names but instead passes on the invitation personally - leaving the researcher blind as to who has been contacted.