Many qualitative research projects have an implicit goal of creating a deeper understanding of a critical issue. As a result, the sampling strategies employed are not concerned with, or designed to allow, generalization of the results to a wider population. Instead, these nonprobabilistic strategies focus on the extent to which the sample chosen provides illuminating information on the phenomenon being studied. The explanations that follow describe some common nonprobability sampling techniques, and are excerpted from, and based upon, Neuman (2007, 141-145).
Haphazard, accidental, or convenience sampling: Identified by many names, this strategy involves a researcher haphazardly selecting potential respondents based solely on the convenience of access to them. This strategy can produce ineffective, highly unrepresentative samples and as a result is not recommended unless no other options are feasible. Commonly encountered examples of haphazard sampling include person-on-the-street interviews and television interviews.
Quota sampling: Requiring a slight modification of haphazard sampling, quota sampling involves a researcher to first identifying relevant categories of people (e.g. male and female) to sample. The researcher than determines a quota to meet in gathering responses from those categories, and accomplishes this task using haphazard methods.
Purposive or judgmental sampling: This strategy is used in situations where a researcher believes some respondents may be more knowledgable than others, and requires an expert to use their judgment in selecting cases with that purpose in mind. The use of judgmental sampling is appropriate in three situations: (1) to select unique respondents who are especially informative; (2) to select members of a difficult-to-reach, specialized population; (3) to identify particular types of respondents for in-depth investigation.
Snowball sampling: Snowball sampling is a method for effectively identifying and sampling the respondents in a network. The crucial feature is that each person or unit is connected with another through a direct or indirect linkage.
Deviant case sampling: Deviant case sampling is similar to purposive sampling in that it is used to seek out respondents that differ from the dominant pattern or that differ from the predominant characteristics of other respondents. This strategy differs from purposive sampling in that the goal is to locate a collection of unusual, different, or peculiar respondents that are not representative of the larger population.
Sequential sampling: A researcher tries to find as many relevant respondents as possible, with the only limit being the exhaustion of relevant respondents or resources. This is similar to judgmental sampling, in that it is entirely dependent upon the judgement of the researcher. When the researcher deems there is no of new information left to be collected, the process is concluded.
Online Resources
Nonprobability Sampling (Research Methods Knowledge Base)
Nonprobability Sampling (Statistics Canada)