Qualitative interview ‘saturation' across four cultural settings: How many interviews are really enough?

To date, theoretical saturation in qualitative interviewing methodology is poorly operationalized in the literature and few publications outline precise steps taken towards ensuring all relevant themes are extracted. In 2006 Guest’s seminal article addressed, for the first time, the paucity in the literature quantitatively defining theoretical saturation for international health research (exploring perceptions of HIV) and concluded that saturation can be reached in the first twelve interviews, regardless of country or study population. This article has been widely cited amongst qualitative researchers to support their sample size and confirm the breadth of information elicited was sufficient. To explore if Guest’s findings held across multiple countries, multiple domains of inquiry, and a more heterogeneous sample, the author employed a re-study of Guest’s approach using qualitative data exploring water insecurity in four countries (Figi, Bolivia, New Zealand, and the southwestern USA). Concepts, codes, and meta-themes were determined and documented in a standardized way that paralleled Guest’s process. Additionally, we varied the ordering of interviews to determine saturation (chronological, reverse chronological, and random). We found that thematic saturation varied depending on location and interview order, ranging from 7 to 18 interviews. Our findings suggest that more precise information is needed to accurately determine the sample size necessary for particular qualitative samples and topics. More studies are needed to better inform qualitative methodologies in international health research.