Short description of application purpose (advantages, disadvantages)

Snowball sampling (also called chain sampling or chain referral) refers to a participant recruitment strategy commonly applied in qualitative research (although to some extent it can also be applicable in quantitative research). It follows the logic of non-probability, purposive methods of sample formation that is characteristic to qualitative research.

Snowball sampling helps recruit research participants that are hard to reach, belong to marginalized, vulnerable or hidden communities, are linked to sensitive contexts or characteristics; or, more generally, researchers access to them is restricted in any other way. If researchers or policy makers want to collect evidence on marginalized or otherwise excluded groups, snowball sampling can be beneficial. The value of the snowball sampling relies on the insider social knowledge. It helps identifying and reaching potential research participants with relevant experiences or characteristics that are “hidden” from those outside the social circle.

Snowball sampling relies on social networks of initial participants (also called initial informants, “seeds” or primary contacts), gradually building on their referrals to next potential participants. Initial informants can also help facilitate researcher’s contact with others in their social network (e.g. by recommending or introducing the researcher). The process works as a snowball game (or chain reaction) where an identified initial participant points to one or two other relevant contacts and some (or all) of them make further referrals. The “chain” builds until the researcher achieves a necessary number and composition of the sample. Among the main challenges related to the snowball sampling are the risk of selection bias and limited control over the selection process. People tend to refer to similar, familiar others, those whom they like or trust. Also, researchers rarely have sufficient insider knowledge about a hidden or hard to reach community (population) and therefore depend on subjective decisions made by initial informants. This can lead to limited diversity and coverage within the recruited sample or entirely overlook individuals who do not belong to any of identified social network (so called “isolates”). There tend to be more control over sample formation in hands of initial informants, their benevolence and action (or non-action) than in hands of the researcher. Moreover, in some cases gaining knowledge about a hidden population and access to initial informants can be substantially time- and labour-consuming.

The following steps guide the process of snowball sampling:

Identifying target population and selection criteria. It is important to clearly define what is the relevant target population and what will be participants’ inclusion and exclusion criteria. The criteria will apply to the selection of initial informants and subsequent referrals. Commonly, the logic of purposive sampling is followed, aiming to select participants that will be most informative with regard to research purpose (based on their experience, socio-demographic or other characteristics).  

Identifying, contacting and selecting initial informants. The next step is to find and contact a small number of initial informants, who meet the selection criteria and are well connected with social networks relevant for the study. To mitigate the risk of bias, it is recommended to look for initial informants from diverse social circles (preferably, initial informants should be as diverse as possible).

Employing social networks and connections to build the referral chain. When contact and rapport with initial informants is established, the next step is to gain referrals to others within their social networks. The researcher asks initial informants to suggest their colleagues, friends, community members or other acquaintances who possibly meet the selection criteria (these criteria have to be clearly communicated to the initial informants to avoid ineligible referrals as much as possible). Trust and recommendation from initial informants are the best facilitators to gain access to relevant pool of participants. Subsequently, the new referrals can be further used as “seed” informants to expand the referral chain.

Filtering through referred contacts and selecting suitable informants.It is important to develop a strategy to filter if referrals do meet the selection criteria (e.g. using a short questionnaire). Likewise, diversifying the social circles of seed informants and proactively looking for isolates helps balancing the final sample.

Continuous evaluation and adjustment of sample formation. It is advised to evaluate sample and reflect on its composition throughout data collection process. For example, after conducting several interviews the researcher may find out about some other community members, sub-groups or contexts that have not been identified before. In such a case, it is possible to initiate a corresponding referral chain to ensure that quality of the final sample. 

Depending on the size and structure of the desired sample, implementation of snowball sampling procedure may become very complicated. It is important to thoroughly register the process – all contacts, seed referral networks and formation of chains, selection decisions, refusals and any other relevant aspects.

Several quality standards for snowball sampling can be highlighted:

Transparency: clear justification of the decision to use snowball sampling and responsible tracking, documenting and reporting of referral chains formation.

Prevention of network homogeneity: diversification of starting points (seeds), constant evaluation of diversity of referral points, control of compliance to selection (inclusion and exclusion) criteria.

Management of ethical risks:  As stated above, snowball sampling is primarily used to reach hidden, marginalized, often vulnerable populations. This requires increased attention to research ethics, specifically, ensuring that participant’s feel safe making referrals within their social circles; ensuring the principle of voluntary participation and preventing any pressures (e.g. peer pressure) to participate; transparently providing option to safely withdraw from the research at any point. Also, using referrals means contacting other people on behalf of the researcher or sharing contacts of other people. Minimising referral contact information to necessary minimum and transparent communication are therefore required.

Atkinson, G. (2025, November 3). Position statement: Ethical considerations in snowball sampling. Iris Ethics. https://www.irisethics.com.au/blog/resources-1/position-statement-ethical-considerations-in-snowball-sampling-24

Atkinson, R., & Flint, J. (2001). Accessing Hidden and Hard-to-Reach Populations: Snowball Research Strategies. Social Research Update, 33. sru.soc.surrey.ac.uk/SRU33.html    

GM Research. (n.d.). What is snowball sampling method? Examples, types, and how to use it. https://tgmresearch.com/snowball-sampling.html

Kirchherr, J., & Charles, K. (2018). Enhancing the sample diversity of snowball samples: Recommendations from a research project on anti-dam movements in Southeast Asia. PLOS ONE, 13(8), e0201710. https://doi.org/10.1371/journal.pone.0201710

Ting, H., Memon, M. A., Thurasamy, R., & Cheah, J.-H. (2025). Snowball sampling: A review and guidelines for survey research. Asian Journal of Business Research, 15(1). https://doi.org/10.14707/ajbr.250186

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