Use of Snowball Sampling to Study Populations Hard - to – Frame

Some populations that we may be interested in studying can be hard-to-frame. These include populations such as of AIDS/HIV positive individuals, individuals/ institutions involved in some illegal activities like theft, burglary, prostitution, use of banned drugs, abortions for sex determination etc. and so forth. Snowball sampling is a non-probability based sampling technique that can be used to gain access to such populations partially up to certain number and then say the findings based on the group selected about such group of individuals.
To have such a sample from the hard-to-frame population, there are two steps namely, i) try and identify one or more sample units in the desired population or render access to such individuals/units; and ii) use these individuals/units to find further similar individuals/units and so on until the required sample size is obtained.
Supposing, the population we are interested in are the students of a university that take banned drugs. Each student may be referred to as a sample unit. Collectively, all students of the university who are such drug users make up our population. However, we are only interested in examining the sample of these drug users who are the students of the university.
Firstly, we need to try and find one or more such students from the university we are concerned. Finding just a small number of individuals willing to identify themselves and take part in the research study on banned drug users may be quite difficult, so the aim is to start with just one or two students.
Due to the sensitivity of the study, the researcher should ask the initial students who agreed to take part in the study to help also in identifying some more students that are also the banned drug users. The process continues until sufficient students of the university have been identified to meet the desired sample size. We need not consider the individuals who are not part of the university at that point of time.
Snowball sampling is a useful choice of sampling strategy when the population is hard-to-frame because:
• It is difficult to identifying individuals/units to include in your sample, perhaps because there is no obvious list/frame of the population you are interested in.
• There may be no other way of accessing/getting your sample, making snowball sampling the only viable choice of sampling strategy.
• The sensitivity of coming forward to take part in a survey is more adverse in such contexts. However, since snowball sampling involve like individuals who know each other and may take part in such a survey as there may be some common characteristics and other social factors between these individuals that help to break down some of the barriers that prevent them from taking part outside their association.
• The unknown nature of some groups may also make it difficult to identify various parts of the population that warrant investigation. In the case of banned drug users, it may be obvious to identify strata such as gender, type of banned drugs used, frequency of the drugs used and so forth. One need to find the characteristics of the population you want to examine at the start of the survey and the same may not be known in its entirety. The snowball sampling may also be helpful in finding the unknown characteristics that could be of interest before settling on your sampling criteria.

Snowball sampling is a not very useful choice of sampling strategy when the population is hard-to-frame if we need to determine the possible sampling error and make generalizations from the sample to the population, since snowball sampling does not select sample units randomly as in case of probability sampling techniques. As such, snowball samples should not be considered to be true representative of the population being studied.