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, J. M. (2010). What is an adequate sample size? Operationalising data saturation for theory-based interview studies. Psychology & Health, 25(10), pp. 1229-1245. doi: 10.1080 This is the unspecified version of the paper.This version of the publication may differ from the final published version. Permanent 2What is an adequate sample size? Operationalising data saturation for theory-based interview studies AbstractIn 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 Theory of Planned Behaviour, designed to identify three belief categories (Behavioural, Normative, Control), using an initial analysis sample of 10 and stopping criterion of 3. Study 1 (retrospective analysis of existing data) identified 84 shared beliefs of 14 general medical practitioners about managing patients with sore throat without prescribing antibiotics. The criterion for saturation was achieved for Normative beliefs but not for other beliefs or study-wise saturation. In Study 2 (prospective analysis), 17 relatives of people with Paget's disease of the bone reported 44 shared beliefs about taking genetic testing. Study-wise data saturation was achieved at interview 17. We propose specification of these principles for reporting data saturation in theory-based interview studies. The principles may be adaptable for other types of studies.

Theory provides a helpful basis for designing interventions to change behaviour but offers little guidance on how to do this. This paper aims to illustrate methods for developing an extensive list of behaviour change techniques (with definitions) and for linking techniques to theoretical constructs. A list of techniques and definitions was generated from techniques published in two systematic reviews, supplemented by "brainstorming" and a systematic search of nine textbooks used in training applied psychologists. Inter-rater reliability of extracting the techniques and definitions from the textbooks was assessed. Four experts judged which techniques would be effective in changing 11 theoretical constructs associated with behaviour change. Thirty-five techniques identified in the reviews were extended to 53 by brainstorming and to 137 by