What Is a Snowball Sample in Sociology?

In sociology, "snowball sampling" refers to a non-probability sampling technique (which includes purposive sampling) in which a researcher begins with a small population of known individuals and expands the sample by asking those initial participants to identify others that should participate in the study. In other words, the sample starts small but "snowballs" into a larger sample through the course of the research.

Snowball sampling is a popular technique among social scientists who wish to work with a population that is difficult to identify or locate. This often occurs when the population is somehow marginalized, like homeless or formerly incarcerated individuals or those who are involved in illegal activities. It is also common to use this sampling technique with people whose membership in a particular group is not widely known, such closeted gay people or bisexual or transgender individuals.

How Snowball Sampling Is Used

Given the nature of snowball sampling, it is not considered a representative sample for statistical purposes. However, it is a very good technique for conducting exploratory research and/or qualitative research with a specific and relatively small population that is hard to identify or locate.

For instance, if you are studying the homeless, it may be difficult or impossible to find a list of all the homeless people in your city. However, if you identify one or two homeless individuals who are willing to participate in your study, they will almost certainly know other homeless individuals in their area and can help you locate them. Those individuals will know other individuals, and so on. The same strategy works for underground subcultures or any population where the individuals prefer to keep their identity hidden, such as undocumented immigrants or ex-convicts.

Trust is an important aspect of any form of research that involves human participants, but it is especially important in a project that requires snowball sampling. For participants to agree to identify other members of their group or subculture, the researcher needs to first develop a rapport and a reputation for trustworthiness. This can take some time, so one must be patient when using the snowball sampling technique on reluctant groups of people. 

Examples of Snowball Sampling

If a researcher wishes to interview undocumented immigrants from Mexico, for example, he or she might interview a few undocumented individuals that he or she knows or can locate, gain their trust, then rely on those subjects to help locate more undocumented individuals. This process continues until the researcher has all the interviews he or she needs or until all contacts have been exhausted. A considerable amount of time is often required for a study that relies on snowball sampling.

If you’ve read the book or have seen the movie "The Help," you will recognize that the main character (Skeeter) uses snowball sampling as she seeks interview subjects for the book she is writing on the conditions for Black women doing housework for white families in the 1960s. In this case, Skeeter identifies one domestic worker who is willing to speak with her about her experiences. That person, Aibileen, then recruits more domestic workers for Skeeter to interview. They then recruit a few more, and so on. In a scientific sense, the method may not have resulted in a representative sample of all African American domestic workers in the South at that time in history, but snowball sampling provided a useful method for qualitative research because of the difficulty finding and reaching out to the subjects.