Encyclopedia of Social Measurement
Snowball sampling is a chain referral method whereby a sample is constructed from a base of initial contacts, who are asked to provide introductions to their associates, who, in turn, are asked to refer others. This process continues until a sample has been built. Snowball sampling is designed for the explicit purpose of obtaining systematic information in situations in which convenience sampling is inappropriate and probability sampling is unrealistic.
Wright, Richard and Stein, Michael, "Snowball Sampling" (2005). Faculty Scholarship. 150.