How we change what others think, feel, believe and do
Use when you do not have access to sufficient people with the characteristics you are seeking.
Find people to study.
Ask them to refer you other people who fit your study requirements, then follow up with these new people.
Repeat this method of requesting referrals until you have studied enough people.
A researcher is studying environmental engineers but can only find five. She asks these engineers if they know any more. They give her several further referrals, who in turn provide additional contacts. In this way, she manages to contact sufficient engineers.
The 'snowball' effect occurs as referrals multiply at each step. For example if you got two referrals from each person, then starting from two people get four more, then eight, sixteen and so on.
Snowball sampling uses a method beloved by sales people, where customer referrals to new prospects have particular value as the relationship of trust and obligation between the identified person and the referrer makes it more likely that the new person will make a purchase.
The way that the sample is chosen by target people makes it liable to various forms of bias. People tend to associate not only with people with the same study selection characteristic but also with other characteristics. This increases the chance of correlations being found in the study that do not apply to the generalized wider population.
The need to get the person to give you a referral also means that the researcher has to form a relationship with the person and be nice to them. This can change the study results as affective biases in both the researcher and the target person change how they think and behave.
With care in selection (you do not have to use every referral) and avoiding personal bias, snowball sampling can still be a useful method, particularly if you have no other way of reaching the target population.
Snowball sampling is also known as chain sampling, chain-referral sampling or referral sampling.