Snowball Sampling

Imagine that you are playing in the winter snow and trying to create large snowballs. When the ball rolls, it collects more snow, getting larger and larger. Think of this metaphorically as a 'snowball effect' - the same process we will explain below. And no, it is not about actual snowballs; it is snowball sampling.

Snowball Sampling Snowball Sampling

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Table of contents
    • To understand what snowball sampling is in psychology, we will start by covering the snowball sampling definition and how exactly snowball sampling in research is used.
    • We will also look at examples of studies that have actually used the sampling technique; this will help you understand the snowball sampling methods in research.
    • To finish off, we will learn about snowball sampling advantages and disadvantages.

    Snowball Sampling: Definition

    So what is snowball sampling?

    Snowball sampling, also known as chain-referral sampling, is a type of sampling where the pool of initially-enrolled participants is used to recruit future participants who meet the required criteria for the study. It is an example of non-probability sampling.

    But what does non-probability sampling mean?

    Non-probability sampling is a method whereby subjects are selected by the researcher using a subjective, non-random technique.

    As well as snowball sampling, there are other types of non-probability sampling, such as convenience sampling; this is when researchers select a sample from a population because they are easily accessible.

    Snowball Sampling in Research

    We may think of snowball sampling as a kind of referral system. Researchers are prone to adopt this sampling technique when the sample size is smaller and less accessible. They ask for assistance in seeking a larger sample size by having their subjects seek out similar ones.

    This sampling method can continue on and on just as a snowball continues to roll. The recruiting process can stop once the researcher has gathered enough data to analyse and draw conclusions on their hypothesis.

    Snowball sampling is also an effective method for researchers when the sample of participants they require may be challenging to find.

    Snowball sampling may be used to find participants with a hidden identity, drug users, or those taking part in illegal activity.

    If you have a small population or a rare disease, snowball sampling will also help find additional participants.

    Snowball Sampling, referring friends for a research study, StudySmarterFigure 1: Snowball sampling relies on participants referring others.

    With snowball sampling, as it is non-probability-based, it must be verified before use to justify why other sampling methods with more reliability and generalisability cannot be used.

    Types of Snowball Sampling

    This sampling method is commonly employed in qualitative research where the group of participants is difficult to locate.

    There are different types of snowball sampling techniques to consider:

    Linear snowball sampling is when the research is determined by a singular, 'linear' sequence of referrals initiated by one participant and goes on. This creates a line of subjects that continues until the researcher is happy with the sample population reached.

    An advantage to this method is that the research is paced and follows easily via a pattern. Despite this, bias remains an issue in this type of snowball sampling.

    Exponential non-discriminative snowball sampling is when the first eligible subject is approached for the study, and then conducts multiple referrals. The chain continues until enough participants are gathered.

    An advantage of this method is that it is faster and provides more options in the sample. However, sometimes, this may lead to variables that do not exactly fit the requirement for the research study.

    Exponential discriminative snowball sampling differs slightly from exponential non-discriminative snowball sampling. The only difference is that this type is discriminative as the researcher screens the participants before accepting them to be involved in the research. The selection of a subject is dependent on the type of study.

    An advantage of this is that this is a more selective process which will provide the best fit for the study. However, this process is time-consuming. The selective nature may also discourage participants from being referred.

    Snowball Sampling Methods

    The researcher calls an initial sample group to participate in the study and asks the subjects to recruit other individuals who they deem fit the criteria. If they are willing, they join the pool of participants.

    The names of potential participants do not need to be shared as a matter of privacy. Individuals are continually recruited until the researcher is satisfied with the sample size.

    The method used depends on the type of snowball sampling method used.

    In exponential non-discriminative snowball sampling, the researcher doesn't screen participants before the experiment, but they do if exponential discriminative snowball sampling is used.

    Snowball Sampling Examples

    Snowball sampling as a technique can be applied to several situations requiring referrals rather than the researcher's direct recruitment of subjects.

    Some individuals may be deterred from being approached by researchers because of stigma.

    You are researching the effects of the drug methamphetamine in a specific population. You are finding it challenging to contact drug users, perhaps because they may be afraid that you are working for the law undercover or fear judgment.

    Using a snowball sampling method to target a small number of individuals for your subject pool could lead you to others.

    In 1987, a study by Kaplan, Korf & Sterk was conducted to investigate the lifestyle of heroin users. They used snowball sampling to understand their social context in two Dutch cities.

    The snowball sampling method is used for social studies. It is a helpful research technique for individuals who may be difficult to identify.

    If you are conducting a study around homelessness in a borough, it may be hard to get in contact with people you need as participants. Yet, if you can reach a few individuals, you can use snowball sampling to grow the number of participants.

    The snowball method is used in a lot of research on specific conditions. It is useful when researching a very rare or difficult condition/situation.

    When researching rare conditions, finding individuals who have them may not be easy. People with the condition are more likely to know others who also have it, so they may be able to help with recruitment.

    Snowball Sampling Advantages and Disadvantages

    There are both advantages and disadvantages to adopting snowball sampling as a technique.

    One of the critical reasons researchers prefer using snowball sampling is that it enables access to the more hidden population that may not be amongst society. It also allows them to gather data without being necessarily upfront or intrusive.

    Researchers save time and resources trying to piece together a sample group because other subjects are effectively responsible for finding and encouraging new participants.

    Snowball sampling also follows a very ethical process. The study is approved by the Board of Institutions to ensure that it is being conducted justifiably and morally. Participants are also not forced to participate; they can accept, decline, or remain anonymous.

    Because the sample is found through referral, this does mean that there is a potential for sampling bias. The whole study is highly reliant on those initial individuals participating. A specific type of person may be more likely to be referred, such as someone more sociable in that particular community. Hence, the conclusive results may not end up being representative of that population.

    Because the sampling method is subjective, it would be hard to determine the sampling error.

    Those involved may also not want to provide details on others participating in the study (protecting their identity or worried about ethical issues).

    Snowball Sampling - Key takeaways

    • Snowball sampling is a type of sampling where the pool of initially-enrolled participants is used to recruit future participants for the study who meet the required criteria.
    • Snowball sampling is an example of non-probability sampling whereby the participants are selected in a subjective way (no random sampling).
    • With snowball sampling, the recruitment of participants can continue until the researcher is satisfied with the pool for their study. It is beneficial to use when the participants required are more hidden in the population and recruitment must occur through other individuals.
    • There are three main types of snowball sampling methods: linear, exponential non-discriminative, and exponential discriminative snowball sampling. The technique is often used in medical, social, or even business psychological studies.
    • An implication that needs to be considered when using sampling bias is sampling bias, which may affect the validity of the results.

    References

    1. Figure 1: Vector created by pikisuperstar on Freepik.
    Frequently Asked Questions about Snowball Sampling

    What is snowball sampling?

    Snowball sampling, also known as chain-referral sampling, is a type of sampling where the pool of initially-enrolled participants is used to recruit future participants who meet the required criteria for the study. It is an example of non-probability sampling. 

    Why do we use snowball sampling?

    Snowball is a kind of referral system used when the sample is less accessible. 

    Is snowball sampling qualitative or quantitative?

    The snowball sampling method is commonly employed in qualitative research where the group of participants may be difficult to recruit. 

    How to determine sample size for snowball sampling?

    There are different snowball sampling methods, and these recruit participants differently. 


    For example, linear snowball sampling is when the research is determined by a singular, 'linear' sequence of referrals initiated by one participant and goes on. This creates a line of subjects that continues until the researcher is happy with the sample population reached. 

    What is the difference between snowball sampling and purposive sampling?

    In purposive sampling, the researcher selects suitable participants for their study based on who they believe is appropriate for their experiment.


    However, in snowball sampling, the sample depends on who the existing research participants identified as potential participants. 

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