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Characterising and justifying sample size sufficiency in interview-based studies: systematic analysis of qualitative healthresearch over a 15-year period

Lookup NU author(s): Dr Susan Thorpe



This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).


Background: Choosing a suitable sample size in qualitative research is an area of conceptual debate and practicaluncertainty. That sample size principles, guidelines and tools have been developed to enable researchers to set,and justify the acceptability of, their sample size is an indication that the issue constitutes an important marker ofthe quality of qualitative research. Nevertheless, research shows that sample size sufficiency reporting is often poor,if not absent, across a range of disciplinary fields.Methods: A systematic analysis of single-interview-per-participant designs within three health-related journals fromthe disciplines of psychology, sociology and medicine, over a 15-year period, was conducted to examine whetherand how sample sizes were justified and how sample size was characterised and discussed by authors. Data pertinentto sample size were extracted and analysed using qualitative and quantitative analytic techniques.Results: Our findings demonstrate that provision of sample size justifications in qualitative health research is limited; isnot contingent on the number of interviews; and relates to the journal of publication. Defence of sample sizewas most frequently supported across all three journals with reference to the principle of saturation and topragmatic considerations. Qualitative sample sizes were predominantly – and often without justification – characterisedas insufficient (i.e., ‘small’) and discussed in the context of study limitations. Sample size insufficiency was seento threaten the validity and generalizability of studies’ results, with the latter being frequently conceived in nomotheticterms.Conclusions: We recommend, firstly, that qualitative health researchers be more transparent about evaluations of theirsample size sufficiency, situating these within broader and more encompassing assessments of data adequacy.Secondly, we invite researchers critically to consider how saturation parameters found in prior methodologicalstudies and sample size community norms might best inform, and apply to, their own project and encouragethat data adequacy is best appraised with reference to features that are intrinsic to the study at hand. Finally,those reviewing papers have a vital role in supporting and encouraging transparent study-specific reporting.

Publication metadata

Author(s): Vasileiou K, Barnett J, Thorpe S, Young T

Publication type: Article

Publication status: Published

Journal: BMC Medical Research Methodology

Year: 2018

Volume: 18

Issue: 18

Online publication date: 21/11/2018

Acceptance date: 14/10/2018

Date deposited: 05/11/2019

ISSN (electronic): 1471-2288

Publisher: BMC


DOI: 10.1186/s12874-018-0594-7


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