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Procedures for estimating sample size in confirmatory survey or experimental research are well established (Cohen 1992). In exploratory research, the theoretical and empirical basis for evaluating sample size is relatively less developed (Onwuegbuzie and Leech 2007) but actively under development. Until recently, all we had were rules of thumb. Morse (1994) proposed sample sizes of 5–50 informants, depending on the purpose of the study. Charmaz (2014) suggested 20–30 for a grounded theory study. Creswell (2007, pp. 126–128) recommended one or two participants in narrative research, 3–10 in phenomenological research, 20–30 in grounded theory research, 4–5 cases in case study research, and “numerous artifacts, observations, and interviews…until the workings of the cultural-group are clear” in ethnography (p. 128).

As Creswell’s advice for ethnographers suggests, a guiding principle is theoretical saturation: Your sample is large enough when you stop getting new information. But how can you estimate in advance how large that will be? Guest et al. (2006) addressed this issue in a study of HIV prevention in Ghana and Nigeria. They interviewed a total of 60 female sex workers. After every six interviews, they tracked which new themes appeared, how frequently each theme occurred, and how much codebook definitions changed. By these measures, Guest et al. reached saturation after only 12 interviews. This finding is consistent with rule-of-thumb guidelines and with predictions from cultural consensus theory (Romney et al. 1986). But Guest et al. note two important caveats. First, the semistructured interview guide was narrowly focused, and all women answered the same questions. In fully unstructured interviews, it would be harder to reach saturation, because new themes would appear as researchers introduced new questions over time. Second, the sample included only one, relatively homogenous subgroup: young, urban, female sex workers. Because sample size is a function of heterogeneity in the phenomenon of interest, adding other subgroups likely would have increased the sample size necessary to reach saturation.