Snowball sampling is hardly likely to lead a representative sample, but there are times when it may be the best or only method available. For instance, if you are studying the homeless, you are not likely to find a list of all the homeless people in your city. However, if you identify one or two homeless individuals that are willing to participate in your study, it is likely that they know other homeless individuals in their area and can help you locate them. The same goes for underground subcultures, or any population that might want to keep their identity hidden, such as undocumented immigrants or ex-convicts.
Because snowball sampling is hardly representative of the larger study population, it is primarily used for exploratory purposes. That is, the researcher is “feeling out” a topic or population to study further in-depth at a later time. Exploratory studies are typically done for three purposes: to satisfy the researcher’s curiosity and desire for better understanding, to test the feasibility of undertaking a more extensive study, and to develop the methods to be employed in any subsequent studies.
If a researcher wishes to interview undocumented immigrants from Mexico, he or she might interview a few undocumented individuals that he or she knows or can locate and would 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.
If you’ve read the book or have seen the movie The Help, you will recognize the main character’s (Skeeter’s) method of finding interview subjects as snowball sampling. In this case, she identified one housekeeper who was willing to speak with her about her experiences working for white families. That housekeeper, Aibileen, then recruited more housekeepers for Skeeter to interview. Those housekeepers then recruited a few more, and so on. It may not have been a representative sample of all black housekeepers in the South, but it was the only method available to Skeeter because of the difficulty finding and reaching out to the subjects.
Babbie, E. (2001). The Practice of Social Research: 9th Edition. Belmont, CA: Wadsworth Thomson.