What is an adequate sample size? Operationalising data saturation for theory-based interview studies

In interview studies, sample size is often justified by interviewing participants until reaching 'data saturation'. However, there is no agreed method of establishing this. We propose principles for deciding saturation in theory-based interview studies (where conceptual categories are pre-established by existing theory). First, specify a minimum sample size for initial analysis (initial analysis sample). Second, specify how many more interviews will be conducted without new ideas emerging (stopping criterion). We demonstrate these principles in two studies, based on the theory of planned behaviour, designed to identify three belief categories (Behavioural, Normative and Control), using an initial analysis…

• SG
  Scottish Government
• CS
  Chief Scientist Office, Scottish Government Health and Social Care Directorate
• MR
  Medical Research Council