Tuesday, 1 November 2011


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.

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