Sampling is the process of selecting a portion of the population, which is an entire aggregate of cases.
In purposive sampling, several strategies have been identiﬁed (Patton, 2002), Patton’s labels his classiﬁcation shows the kind of diverse strategies qualitative researchers have adopted to meet the theoretical needs of their research-
- Maximum variation sampling involves purposefully selecting cases with a wide range of variation on dimensions of interest. By selecting participants with diverse views and perspectives, researchers invite challenges to preconceived or emerging conceptualizations. Maximum variation sampling might involve ensuring that people with diverse backgrounds are represented in the sample (ensuring that there are men and women, poor and afﬂuent people, and so on). It might also involve deliberate attempts to include people with different viewpoints about the phenomenon under study. For example, researchers might use snowballing to ask early participants for referrals to people who hold different points of view.
- Homogeneous sampling deliberately reduces variation and permits a more focused inquiry. Researchers may use this approach if they wish to understand a particular group of people especially well. Homogeneous sampling is often used to select people for group interviews.
- Extreme (deviant) case sampling provides opportunities for learning from the most unusual and extreme informants (e.g., outstanding successes and notable failures). The assumption underlying this approach is that extreme cases are rich in information because they are special in some way. In some cases, more can be learned by intensively studying extreme cases, but extreme cases can also distort understanding of a phenomenon.
- Intensity sampling is similar to extreme case sampling but with less emphasis on the extremes. Intensity samples involve information-rich cases that manifest the phenomenon of interest intensely, but not as extreme or potentially distorting manifestations. Thus, the goal in intensity sampling is to select rich cases that offer strong examples of the phenomenon.
- Typical case sampling involves the selection of participants who illustrate or highlight what is typical or average. The resulting information can be used to create a qualitative proﬁle illustrating typical manifestations of the phenomenon being studied.
- Critical case sampling involves selecting important cases regarding the phenomenon of interest. With this approach, researchers look for the particularly good story that illuminates critical aspects of the phenomenon.
- Criterion sampling involves studying cases that meet a predetermined criterion of importance. Criterion sampling is sometimes used in multi method studies in which data from the quantitative component are used to select cases meeting certain criteria for in-depth study. Sandelowski (2000) offers a number of helpful suggestions for combining sampling strategies in mixed-method research
- Theory-based sampling involves the selection of people or incidents on the basis of their potential representation of important theoretical constructs. Theory-based sampling is a much focused approach that is usually based on an a priori theory that is being examined qualitatively.
- Sampling conﬁrming and disconﬁrming cases is often used toward the end of data collection in qualitative studies. As researchers note trends and patterns in the data, emerging conceptualizations need to be checked. Conﬁrming cases are additional cases that ﬁt researchers’ conceptualizations and offer enhanced credibility. Disconﬁrming cases are examples that do not ﬁt and serve to challenge researchers’ interpretations. These “negative” cases may offer new insights about how the original conceptualization needs to be revised or expanded.