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

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          Abstract

          Background

          Choosing a suitable sample size in qualitative research is an area of conceptual debate and practical uncertainty. 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 of the 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 from the disciplines of psychology, sociology and medicine, over a 15-year period, was conducted to examine whether and how sample sizes were justified and how sample size was characterised and discussed by authors. Data pertinent to 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; is not contingent on the number of interviews; and relates to the journal of publication. Defence of sample size was most frequently supported across all three journals with reference to the principle of saturation and to pragmatic considerations. Qualitative sample sizes were predominantly – and often without justification – characterised as insufficient (i.e., ‘small’) and discussed in the context of study limitations. Sample size insufficiency was seen to threaten the validity and generalizability of studies’ results, with the latter being frequently conceived in nomothetic terms.

          Conclusions

          We recommend, firstly, that qualitative health researchers be more transparent about evaluations of their sample 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 methodological studies and sample size community norms might best inform, and apply to, their own project and encourage that 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.

          Electronic supplementary material

          The online version of this article (10.1186/s12874-018-0594-7) contains supplementary material, which is available to authorized users.

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          Most cited references34

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          What about N? A methodological study of sample-size reporting in focus group studies

          Background Focus group studies are increasingly published in health related journals, but we know little about how researchers use this method, particularly how they determine the number of focus groups to conduct. The methodological literature commonly advises researchers to follow principles of data saturation, although practical advise on how to do this is lacking. Our objectives were firstly, to describe the current status of sample size in focus group studies reported in health journals. Secondly, to assess whether and how researchers explain the number of focus groups they carry out. Methods We searched PubMed for studies that had used focus groups and that had been published in open access journals during 2008, and extracted data on the number of focus groups and on any explanation authors gave for this number. We also did a qualitative assessment of the papers with regard to how number of groups was explained and discussed. Results We identified 220 papers published in 117 journals. In these papers insufficient reporting of sample sizes was common. The number of focus groups conducted varied greatly (mean 8.4, median 5, range 1 to 96). Thirty seven (17%) studies attempted to explain the number of groups. Six studies referred to rules of thumb in the literature, three stated that they were unable to organize more groups for practical reasons, while 28 studies stated that they had reached a point of saturation. Among those stating that they had reached a point of saturation, several appeared not to have followed principles from grounded theory where data collection and analysis is an iterative process until saturation is reached. Studies with high numbers of focus groups did not offer explanations for number of groups. Too much data as a study weakness was not an issue discussed in any of the reviewed papers. Conclusions Based on these findings we suggest that journals adopt more stringent requirements for focus group method reporting. The often poor and inconsistent reporting seen in these studies may also reflect the lack of clear, evidence-based guidance about deciding on sample size. More empirical research is needed to develop focus group methodology.
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            Assessing and demonstrating data saturation in qualitative inquiry supporting patient-reported outcomes research.

            In the patient-reported outcomes (PROs) field, strict regulatory requirements must be met for qualitative research that contributes to labeling claims for medicinal products. These requirements not only emphasize the importance of reaching saturation but also of providing documentary evidence that saturation has been reached. This paper reviews qualitative literature for useful definitions of the concept and for practical approaches for assessing saturation. The paper considers approaches in light of the rigorous regulatory requirements for PRO research that are used to support labeling claims for medicinal products and the wider requirements for flexibility and creativity in qualitative research in general. This assessment is facilitated by the use of examples from our past qualitative PRO studies. Based on conclusions from this assessment, we offer preliminary recommendations for future qualitative PRO studies for assessing and documenting saturation.
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              "Data were saturated . . . ".

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                Author and article information

                Contributors
                +44 (0) 1225 383167 , K.Vasileiou@bath.ac.uk
                J.C.Barnett@bath.ac.uk
                susan.thorpe@newcastle.ac.uk
                terry.young@brunel.ac.uk
                Journal
                BMC Med Res Methodol
                BMC Med Res Methodol
                BMC Medical Research Methodology
                BioMed Central (London )
                1471-2288
                21 November 2018
                21 November 2018
                2018
                : 18
                Affiliations
                [1 ]ISNI 0000 0001 2162 1699, GRID grid.7340.0, Department of Psychology, , University of Bath, ; Building 10 West, Claverton Down, Bath, BA2 7AY UK
                [2 ]ISNI 0000 0001 0462 7212, GRID grid.1006.7, School of Psychology, , Newcastle University, ; Ridley Building 1, Queen Victoria Road, Newcastle upon Tyne, NE1 7RU UK
                [3 ]ISNI 0000 0001 0724 6933, GRID grid.7728.a, Department of Computer Science, , Brunel University London, ; Wilfred Brown Building 108, Uxbridge, UB8 3PH UK
                Article
                594
                10.1186/s12874-018-0594-7
                6249736
                30463515
                3fa4fcd8-4081-41f9-8587-2e66a4b5777f
                © The Author(s). 2018

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License ( http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver ( http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

                Funding
                Funded by: FundRef http://dx.doi.org/10.13039/501100000266, Engineering and Physical Sciences Research Council;
                Award ID: EP/F063822/1 and EP/G012393/1
                Award Recipient :
                Categories
                Research Article
                Custom metadata
                © The Author(s) 2018

                Medicine
                sample size,sample size justification,sample size characterisation,data adequacy,qualitative health research,qualitative interviews,review,systematic analysis

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