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Many medical anthropologists also use purposive sampling techniques. The goal of purposive sampling is to represent important dimensions of variation relevant to the aims of research. There are many approaches to purposive sampling, including selection of extreme, typical, unique, or politically important cases; selection to maximize homogeneity or heterogeneity of the sample; and identification of critical cases who have specialized knowledge or experiences relevant to the subject of interest (Onwuegbuzie and Leech 2007). In ethnographic research, the selection of key informants is an example of critical-case sampling.

Medical anthropologists often combine the building blocks in ssss1 to construct complex, multistage sampling designs. Baer et al. (2003) used a two-stage sampling design in their study of cross-cultural differences and similarities in the meaning of the folk illness nervios in Mexico, Guatemala, and the United States. In each of four sites, they purposively selected clusters – “a village, neighborhood, or census tract” (p. 319) – based on differences in social class, ethnicity, and other factors. Then they randomly selected roughly 40 households from each site, for a total sample size of 158.