SAMPLE SIZE IN NONPROBABILITY SAMPLING
There is growing evidence that 10-20 knowledgeable people are enough to uncover and understand the core categories in any well-defined cultural domain or study of lived experience. Morse (1994) recommended a minimum of six interviews for phenomenological studies and 30-50 interviews for ethnographic studies and grounded theory studies. The data from two recent studies support Morse’s experience-based guess.
M. G. Morgan et al. (2002:76) did in-depth interviews with four different samples of people about various risks in the environment. As they coded the interviews for concepts, Morgan et al. plotted the number of new concepts in each interview across the four samples. In all four cases, the first few interviews produce a lot of new data, but by 20 interviews, hardly any new information is retrieved.
Guest et al. (2006) interviewed 30 sex workers in Ghana and another 30 in Nigeria. They coded the transcripts in batches of six, working first on the interview from Ghana and then on the ones from Nigeria, and plotted the number of new themes uncovered in the coding. Of the 114 themes identified in the entire corpus, 80 turn up in the first six interviews in both Ghana and Nigeria. Another 20 themes turned up in the second batch of six interviews. Only 5 new themes were added to the codebook to accommodate the 30 interviews from Nigeria.
And, as we’ll see in tables 16.13 and 16.14 (when we get to consensus analysis), Weller and Romney (1988:77) showed that just 10-13 knowledgeable informants are needed to understand the contents of a well-defined cultural domain. This is all very good news for ethnographers (and see Handwerker 2001:93-96).