As outlined in the general principles of the chapter on interviewing, sampling in qualitative research is seldom statistically based. Also samples of settings or groups to observe are purposive.
Specifically for observation the sampling units are places, locations, and blocks of time, but usually not individuals. The aim is to select ‘information-rich’ cases, but in practice site selection is often a pragmatic decision based on existing networks and accessibility. Ideally however, sites are chosen because they typify some larger population of sites (such as clinics) or perhaps because they are exceptional in some way. Observation methods may be used across multiple sites and one could select the ones representing a range of typical settings (Green et al, 2009).