Definition: The Snowball Sampling is a non-random sampling technique wherein the initial informants are approached who through their social network nominate or refer the participants that meet the eligibility criteria of the research under study. Thus, this method is also called as the referral sampling method or chain sampling method.
The snowball sampling method is extensively used in the situations when the population is unknown and rare, and it is hard to select the subjects therefrom. First-of-all the initial informants (acquaintances) are contacted who further give the reference of other people whom they think will fit best for the research study, and then they are contacted to get the insights and knowledge about the research being conducted.
For example, the group of people suffering from AIDS is limited and often reluctant to disclose their disease. And in such case, if the interviewer wants to know how the life of these people have changed due to AIDS, might approach those acquaintances who can refer those individuals who can potentially contribute to the study.
In snowball sampling, the initial subject gives a link to other subject and likewise the chain of respondents gets created. Hence, the success of this method depends purely on the initial subject which gives further references.