In statistical research, snowball sampling is a non-probability method used for rare studies or research limited to rare small groups of population. The technique works like a chain referral system where existing study subjects recruit additional subjects from their contact groups. This causes the sample group to keep growing similar to how a snowball grows. Snowball sampling is also referred to as chain sampling, chain referral sampling or referral sampling. Once the original subject has been studied, the researcher asks the subject to find people with similar traits to expand the sampling group.
Useful for Sampling of Special Population Segments
In some cases, sampling of members of a special population segment are require, but prove difficult to locate. In such cases, snowball sampling is the best sampling method to choose. For example, studies on Mexican or other nationality migrants in a particular city in America can be done through snowball sampling. One subject is identified and studied and then asked to recruit other subjects from the person's acquaintances and contacts. This sampling method is also useful when researching groups, for example a specific recruitment pattern in an organisation.
Snowball sampling method is generally used for low incidence or rare population groups, where sampling poses a big problem. When the defined population with the sample is unavailable, this method can prove effective in finding the sample subjects. It involves finding one subject and researching the subject. Then, the subject is used to find more sample subjects through friends, family and acquaintances. In this case the cost of locating samples and researching is not very high. The researcher is not spending time and money trying to find the sample subjects; rather they are being brought to the researcher.
Snowball sampling is based on researching one subject and using the subject to recruit more members for sampling. These people are known to the initial subject, who is more than likely to nominate people they know very well. These people will more often than not share similar traits and behaviour characteristics. What the sampler may end up with is a small subgroup of the entire population. Since this sampling method is used predominantly in race sampling, one subject can very well nominate an entire family, close friends and other acquaintances. All of whom may exhibit the same traits and characteristics, leading to sampling bias.
Lack of Control over the Sampling Method
In the snowball sampling method, the researcher actually has very little control over the sampling method. The type of subjects that the researcher can secure for sampling is mainly dependent on the original subjects that were researched. After the first set of original subjects is researched, the researcher may lose control over the sampling method. The reason being the original subjects are tasked with adding to the sampling pool by nominating people they know. Another problem is that representatives of the sample are not guaranteed because the researcher has no idea of the true distribution of the sample.