In which sampling method do existing subjects recruit future subjects?

Snowball sampling is a unique qualitative research method particularly useful when studying hard-to-reach populations. In this method, existing subjects—also known as participants—play a crucial role in recruiting future subjects. Once initial subjects are identified, they help researchers find additional subjects who share similar characteristics or experiences. This process continues, creating a "snowball" effect as more subjects are recruited based on referrals from those already involved in the study. This method is especially effective when there is no accessible list of the entire population or when individuals are reluctant to participate in research studies.

The other options describe different sampling techniques. Cluster sampling involves dividing the population into clusters (often geographically) and randomly selecting entire clusters for study, rather than relying on individual referrals. Judgment sampling is where researchers select subjects based on their judgment about who would be best for the study, not through participant referral. Quota sampling entails meeting a pre-set number of subjects from different subgroups within the population based on certain characteristics, again without relying on existing subjects to recruit new ones. Each of these methods serves different research needs and contexts, highlighting the specific purpose and strengths of snowball sampling in qualitative research settings.