I am regularly asked by research students: what is an appropriate sample size for a qualitative research study?
The answer is: it depends
There is no exact way of determining sample size in qualitative research, nor a ‘right’ answer in the same way a power calculation may yield a sample size in quantitative research. According to Morse (2000, p.3) sample size depends on consideration of a number of factors including:
“the quality of data, the scope of the study, the nature of the topic, the amount of useful information obtained from each participant, the number of interviews per participant, the use of shadowed data, and the qualitative method and study design used.”
Morse makes the following observations: a study that is broad in scope may require greater number of participants than one that is narrower in focus. A phenomenon that is more difficult to grasp and is below the surface would require more participants. The quality of the data in terms of richness, experiences and relatedness to the research questions is important to consider – the more usable data you have the less participants are needed.
For me there are two key dimensions that need to be considered when thinking about sample size. One dimension is breadth versus depth: so that a phenomenological study that has repeat engagement with participants and an in-depth immersive element would require less participants than say a grounded theory study with single data collection points per participant. So this links with study design as outlined by Morse above. Another important dimension is pragmatic. Is this study part of an Honours, Masters or PhD thesis where size and scope are to some extent imposed? Is the study funded? Is it single site or multi-site? Are there multiple data sources and methods of data collection? How many years will you invest in this study? What is adequate for answering the research question?
What might be becoming apparent here is that often you cannot estimate the number of participants needed in advance of the study – yet a range needs to be specified when applying to ethics. The decision of how many participants within that range then falls down to a consideration of all the factors explained above. That’s why talking to someone with experience in qualitative research and looking at different sampling strategies in your research field and your methodological choice are so important.
The elephant in the room not yet touched upon is that god-term ‘saturation’. How many articles have you read that claim saturation as the defense for their sample size? I know I’ve made that claim in at least one of my published articles (cringe). What does it mean – that no themes ‘emerge’ (another pet-hate – last I checked data analysis was a very active interpretive process? but I digress) from the data. Saturation is used as a ‘marker for sampling adequacy’ (O’Reilly and Parker 2013). Can you achieve saturation in qualitative research? Perhaps the next person you sample would have a very different experience? Perhaps a different lens or deeper analysis could identify other understandings and meanings related to the experience? In their paper O’Reilly and Parker (2013) critique the concept of saturation in qualitative research. They claim that theoretical saturation in grounded theory research is epistemologically robust:
“In grounded theory the notion of saturation does not refer to the point at which no new ideas emerge, but rather means that categories are fully accounted for, the variability between them are explained and the relationships between them are tested and validated and thus a theory can emerge (Green and Thorogood, 2004).” Cited in (O’Reilly and Parker 2013, p. 3)
The problem is when such a meaningful, situated term is removed from its context and applied in other qualitative research unquestioningly and lacking in transparency. This can be a problem with the use of many terms in qualitative research e.g. thematic analysis was done. One of the things I find very liberating about qualitative research is the lack of rules and protocols (or straitjackets if you will). But to do it justice then a rigorous understanding of philosophical frameworks, embedded traditions and accepted practices is needed and more importantly the ability to defend what you do in a transparent and academically robust way. I realise that the uncertainty around what you can and can’t do in qualitative research can be quite daunting for new researchers but it is also what makes qualitative research interesting, creative and fun!
Morse, J. M. (2000). Determining Sample Size. Qualitative Health Research, 10(1), 3-5.
O’Reilly, M., & Parker, N. (2013). ‘Unsatisfactory Saturation’: a critical exploration of the notion of saturated sample sizes in qualitative research. Qualitative Research, 13(2), 190-197.