Data collection and sampling in qualitative research: does size matter?

In this editorial we discuss issues relating to gaining quality data from individual and focus group interviewees. The primary strength of qualitative research is its potential to explore a topic in depth (Carlsen too few may risk adequate depth and breadth, but too many may produce superficial or unwieldy volumes of data (Sandelowski 1995, p. 179). For example, an experienced interviewer, with a clearly defined research topic, and a small number of well-selected homogeneous interviewees (with adequate exposure to or experience of the phenomenon) can produce highly relevant information for analysis. An inexperienced interviewer with a variable and very large sample could result in superficial data, providing a false sense of security and/or generating large amounts of information non-conducive to in-depth analysis. It is, therefore, important that qualitative researchers justify their sample size on the grounds of quality data – something that should be clearly reflected in the presentation of the study's findings. It is also important that, at least, the majority of participants are represented in the presentation of data. It is not uncommon to see papers that report a sample of 20, but on close inspection data are actually presented from less than half of the sample – did the others not say anything? If sample size is not central to qualitative research as such, what criteria are used to cease interviewing? The key difference between quantitative (where the term sample size originates) and qualitative research is that the latter is an interactive process via which information – some of which will become ‘data’ – is likely to emerge (e.g. individual and focus group interviews). Stopping information gathering is dependent on ‘redundancy’ of information or ‘saturation’. Redundancy is ‘the process of sequentially conducting interviews until all concepts are repeated multiple times without new concepts or themes emerging’ (Trotter 2012, p. 399). That is, analysis is carried out after each interview and when the researcher finds the conceptual wellspring has dried up and interviewees reiterate each others’ ideas, one way or another, redundancy has been achieved. Saturation is reached when ‘all questions have been thoroughly explored in detail and no new concepts or themes emerge in subsequent interviews’ (Trotter 2012, p. 399). The term saturation initially arose from grounded theory, which is a complex and demanding qualitative research methodology; however, in recent years a significant proportion of qualitative researchers have taken up the term without adhering to its stringent methods of concurrent data collection and analysis for theory development (Carlsen benefit from group interactivity; prevent socially correct responses; avoid non-conducive activities such as the researcher holding the floor; or stop participants feeling shy about contradicting a fellow interviewee? At the other end of the scale, who amongst us has the skills to manage in-depth focused information gathering from more than twelve participants (even with a digital recorder/note taker) who are unknown to each other, within a timeframe of 90 minutes? Fundamentally, for focus group methods of data gathering the phenomenon being investigated should determine how many participants in each group to optimize interactivity; and the research question itself guides researchers to decide how many focus groups are needed and why. The use of 3–5 focus groups is well-documented (Twohig & Putnam 2002), but this may turn out to be another oft repeated rule-of-thumb that avoids rigorous questions about what, who and why. To facilitate adequate data collection and the emergence of themes across and between groups, fewer than 2 groups raises the question of true representation (Halcomb et al. 2007), if providing extensiveness and representativeness (borrowed from quantitative research principles) are pertinent aims. Accurate reporting of qualitative research methods during all stages is important for reader comprehension and transparency. When discussing focus group interviews, Twohig and Putnam (2002) consider that recruitment details, the range of the groups and a description of the participants, along with inclusion and exclusion criteria should all be clearly documented. Readers and reviewers should be able to work out what was done, how it was done and why, from the published manuscript. This includes reporting unexpected practical adaptations. In their review, Carlsen and Glenton (2011) found inconsistent method and analysis documentation in published qualitative research, as well as a lack of evidence of rigour. Consequently they suggest that journal editorial teams could benefit from using guidelines for reporting qualitative methods, such as RATS (see http://www.biomedcentral.com/info/ifora/rats0). In this editorial we have explored a range of issues around qualitative research planning and participant selection for individual and focus group interviews. Interviewer skill, reasons for including and excluding participants, and the connection between data analysis and the number of interviewees, and transparent reporting of decision-making have also been addressed.