PURPOSIVE SAMPLING

PURPOSIVE SAMPLING - Subjects are selected because of some characteristic. Patton (1990) has proposed the following cases of purposive sampling. Purposive sampling is popular in qualitative research.

• Extreme or Deviant Case - Learning from highly unusual manifestations of the phenomenon of interest, such as outstanding success/notable failures, top of the class/dropouts, exotic events, crises.
• Intensity - Information-rich cases that manifest the phenomenon intensely, but not extremely, such as good students/poor students, above average/below average.
• Maximum Variation - Purposefully picking a wide range of variation on dimensions of interest...documents unique or diverse variations that have emerged in adapting to different conditions. Identifies important common patterns that cut across variations.
• Homogeneous - Focuses, reduces variation, simplifies analysis, facilitates group interviewing.
• Typical Case - Illustrates or highlights what is typical, normal, average.
• Stratified Purposeful - Illustrates characteristics of particular subgroups of interest; facilitates comparisons.
• Critical Case - Permits logical generalization and maximum application of information to other cases because if it's true of this once case it's likely to be true of all other cases.
• Snowball or Chain - Identifies cases of interest from people who know people who know people who know what cases are information-rich, that is, good examples for study, good interview subjects.
• Criterion - Picking all cases that meet some criterion, such as all children abused in a treatment facility. Quality assurance.
• Theory-Based or Operational Construct - Finding manifestations of a theoretical construct of interest so as to elaborate and examine the construct.
• Confirming or Disconfirming - Elaborating and deepening initial analysis, seeking exceptions, testing variation.
• Opportunistic - Following new leads during fieldwork, taking advantage of the unexpected, flexibility.
• Random Purposeful - (still small sample size) Adds credibility to sample when potential purposeful sample is larger than one can handle. Reduces judgment within a purposeful category. (Not for generalizations or representativeness.)
• Politically Important Cases - Attracts attention to the study (or avoids attracting undesired attention by purposefully eliminating from the sample politically sensitive cases).
• Convenience - Saves time, money, and effort. Poorest rational; lowest credibility. Yields information-poor cases.
• Combination or Mixed Purposeful - Triangulation, flexibility, meets multiple interests and needs. (Patton, 1990)

Patton, M. Q. (1990). Qualitative evaluation and research methods (2nd ed.). Newbury Park, CA: Sage Publications.