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Snowball sampling is where research participants recruit other participants for a test or study. It is used where potential participants are hard to find. It’s called snowball sampling because (in theory) once you have the ball rolling, it picks up more “snow” along the way and becomes larger and larger. Snowball sampling is a non-probability sampling method. It doesn’t have the probability involved, with say, simple random sampling (where the odds are the same for any particular participant being chosen). Rather, the researchers used their own judgment to choose participants.
Snowball sampling consists of two steps:
- Identify potential subjects in the population. Often, only one or two subjects can be found initially.
- Ask those subjects to recruit other people (and then ask those people to recruit. Participants should be made aware that they do not have to provide any other names.
These steps are repeated until the needed sample size is found. Ethically, the study participants should not be asked to identify other potential participants. Rather, they should be asked to encourage others to come forward. When individuals are named, it’s sometimes called “cold-calling”, as you are calling out of the blue. Cold-calling is usually reserved for snowball sampling where there’s no risk of potential embarrassment or other ethical dilemmas. For example, it would be easier to cold-call participants in a study for families who regularly dine at fast-food restaurants than it would be to cold-call people who are having extra-marital affairs.
Snowball sampling can be a tricky ethical path to navigate. Therefore, you’ll probably be in contact with an institutional review board (like this one http://research.boisestate.edu/compliance/institutional-review-board-irb-home/ ) or another department similarly involved in ethics.
Why is Snowball Sampling Used?
Some people may not want to be found. For example, if a study was investigating cheating on exams, shoplifting, drug use, prostitution, or any other “unacceptable” societal behavior, potential participants would be wary of coming forward because of possible ramifications. However, other study participants would likely know other people in the same situation as themselves and could inform others about the benefits of the study and reassure them of confidentiality.
Advantages and Disadvantages of Snowball Sampling
- It allows for studies to take place where otherwise it might be impossible to conduct because of a lack of participants.
- Snowball sampling may help you discover characteristics about a population that you weren’t aware existed. For example, the casual illegal downloader vs. the for-profit downloader.
- It us usually impossible to determine the sampling error or make inferences about populations based on the obtained sample.
Snowball sampling is also known as cold-calling, chain sampling, chain-referral sampling, and referral sampling.
Everitt, B. S.; Skrondal, A. (2010), The Cambridge Dictionary of Statistics, Cambridge University Press.
Levine, D. (2014). Even You Can Learn Statistics and Analytics: An Easy to Understand Guide to Statistics and Analytics 3rd Edition. Pearson FT Press.