Evaluation: The Tipping Point - A new type of case?

In Purposive Sampling subjects are selected because of some characteristic. Michael Quinn Patton (1990) has proposed the following cases of purposive sampling.

1. 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.

2. Intensity - Information-rich cases that manifest the phenomenon intensely, but not extremely, such as good students/poor students, above average/below average.

3. 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.

4. Homogeneous - Focuses, reduces variation, simplifies analysis, and facilitates group interviewing.

5. Typical Case - Illustrates or highlights what is typical, normal, average.

6. Stratified Purposeful - Illustrates characteristics of particular subgroups of interest; facilitates comparisons.

7. 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.

8. 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.

9. Criterion - Picking all cases that meet some criterion, such as all children abused in a treatment facility. Quality assurance.

10. Theory-Based or Operational Construct - Finding manifestations of a theoretical construct of interest so as to elaborate and examine the construct.

11. Confirming or Disconfirming - Elaborating and deepening initial analysis, seeking exceptions, testing variation.

12. Opportunistic - Following new leads during fieldwork, taking advantage of the unexpected, flexibility.

13. 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.)

14. Politically Important Cases - Attracts attention to the study (or avoids attracting undesired attention by purposefully eliminating from the sample politically sensitive cases).

15. Convenience - Saves time, money, and effort. Poorest rational; lowest credibility. Yields information-poor cases.

16. Combination or Mixed Purposeful - Triangulation, flexibility, meets multiple interests and needs. (Patton, 1990)

I was thinking about these cases when I recently re-read Malcolm Gladwell’s book, The Tipping Point. In this book he said: "The Law of The Few says that there are exceptional people out there who are capable of starting an epidemic. All you have to do is find them." (Page 132). Hence, he suggests, in so many words, a “Tipping Point” case. But wait, these cases can be further broken down, (and are, by Gladwell) to the following:

1. Connectors - people who "link us up with the world ... people with a special gift for bringing the world together." – think Kevin Bacon….

2. Mavens - “information brokers" or people who connect us with new information – I think of bloggers…..

3. Salesmen are "persuaders", often persons with powerful negotiation skills – Barack Obama?

These persons are different from being extreme cases or critical cases, under Patton’s definition. They certainly fit “cases” one might consider in mathematical sociology, social network analysis, or case studies. So has Gladwell identified additional cases that should added as part of the literature to the type of cases evaluators consider today when sampling?