What sample size is used in qualitative research?

All marketing practitioners who have taken up consumer research would have encountered the question – how big should the sample size be? For qualitative studies, the answer typically is: “As big a sample size as you can afford to have. The more the better!”. Auris takes the ‘sample size’ out of the equation and makes this question redundant. Imagine qualitative research with significantly large, unprompted views of the consumers, rather than just a handful of views. For those interested in research techniques, here’s some more detail you might find interesting.

When deciding the sample size for a qualitative market research project:

• Do we follow Patton (2002) by accepting that even 1 case is enough for research which does not have theory-building as core focus, and plans only to explore an issue or offer depth to quantitative data? After all, according to Patton (1990), a researcher needs to focus on what is doable or feasible; because sampling to the point of redundancy requires unlimited timelines as well as unlimited resources.
• Or, do we follow Saunders, Sim et al. (2012) who ask us to stop sampling only after we reach theoretical or conceptual saturation, if we aim to build a theory? If saturation makes further data collection unnecessary, we need to ensure that its operationalization is consistent with the research question, the theoretical position and the analytic framework adopted.

Let’s consider the issues involved.

• Qualitative research indicates that it does not rely on any quantitative necessities, like sample size. Typically, it requires smaller sample sizes than quantitative analysis would.
• Sample size in qualitative data analysis is measured by the number of themes or categories identified within the data.
• Saturation is the point where adding new data does not improve the explanations of the themes or the categories or add any new perspectives or information.
• Diminishing returns on qualitative data occur when more data leads to no new information, as one occurrence is enough to add the information to the analytical framework.
• Gathering too much data and too many samples could make a research project impractical and time consuming, as each new piece of data brings in additional complexities.
• It is important to capture all perceptions, but there is no need to do it on a repetitive basis, without adding any additional inputs to the research points being inquired into.

So, we accept that it is prudent for a qualitative researcher to take up enough cases to reach a saturation point where additional efforts will not be able to yield further useful information from the sample.

More about reaching saturation and why the sample size is important:

• The size of a sample and the design of the project determine the time taken for a project to reach saturation. To illustrate, a study covering Waldorf education in India up to the seventh standard would naturally take less time to reach saturation than a study of education in general.
• The amount of information obtained will also depend upon the interviewing techniques and skills of the interviewer.
• Some projects have large sample sizes because:
  • The sample has heterogeneous population
  • The project uses multiple selection criteria
  • The study contains multiple samples
  • The project needs an extensive ‘nesting’ of criteria
  • The sample contains groups of special interest requiring intensive study.
• There’s no scientific explanation or method for determining that saturation has been reached at a specific point in the survey.
• Saturation is considered to be a matter of degree by some, with the potential for some new data to emerge.

Various factors decide the sample sizes in qualitative studies, and these suggestions range from 5 to 60, with 30, 40 and 50 being other significant reference points. Ultimately, the decision to call a halt to the study can be taken by the researchers, when they finally find that with each new interview they are unable to extract any significant new information to add to their data bank. That would prove to be the ideal sample size for their qualitative research study.

This is where an AI-powered social listening tool like Auris offers a market research team the unique advantage of being able to listen to an unlimited amount of feedback in real time. What’s more, these opinions, views or feedback are as random and unprompted as they can be, yielding highly dependable predictive analyses as the subjectivity which researchers tend to bring in is eliminated completely.

The question of how many participants are enough for a qualitative interview is, in my opinion, one of the most difficult questions to find an answer to in the literature. In fact, many authors who set out to find specific guidelines on the ideal sample size in qualitative research in the literature have also concluded that these are “virtually non-existent” (Guest, Bunce and Johnson, 2005: 59).   This is particularly unfortunate, given that as a student planning to undertake your research, one of the things that will be most likely to be asked of you is to indicate, and justify, the number of participants in your planned study (this also includes your PhD proposal in which you are expected to give as much detail of the study as possible).

If you, then, turn to the literature, hoping to find advice from some of the great minds in research methodology, you are likely to find them evading the question and often hiding behind the term “saturation” which refers to the point at which gathering new data does not provide any new theoretical insights into the studied phenomenon. Although the concept of saturation may also be controversial, not least because the longer you explore, analyse and reflect on your data, you are always likely to find something “new” in it, it has come to be the guiding concept in establishing sample size in many qualitative studies. As Guest, Bunce and Johnson (2005) rightly point out, however

“although the idea of saturation is helpful at the conceptual level, it provides little practical guidance for estimating sample sizes for robust research prior to data collection”

     (Guest, Bunce and Johnson, 2005: 59)

In other words – how in the world are we supposed to know when we will reach saturation PRIOR TO THE STUDY???

My advice is to use the available literature on the point of saturation and use it to justify your decision regarding the sample size. I did it for my PhD study, as I was growing frustrated that I really have to justify my decision to include 20 participants for an interview, even though I had read dozens of reports in which this number, or smaller, was common (“are you going to interview 20 participants just because others did?”). I just felt that this would be enough, and my common sense, which as I learnt throughout my PhD was the last thing that anyone would care about, was telling me the same thing. In order to support my decision with the literature, however, and considering that there are hardly any guidelines for establishing sample size, I decided to try to reach some sort of conclusion as to how many participants are enough to reach saturation and use it as my main argument for establishing the size of the sample.

So what does the literature tell us about this? Just as there is not single answer as to what sample size is sufficient, there is no single answer to the question of what sample size is sufficient to reach theoretical saturation.  Such factors as heterogeneity of the studied population, the scope of the study and the adopted methods and their application (e.g. the length of the interviews) are believed, however, to have a central role in achieving this (cf. Baker and Edwards, 2012; Guest, Bunce and Johnson, 2005; Mason, 2010). Mason’s (2010) analysis of 560 PhD studies that adopted a qualitative interview as their main method revealed that the most common sample size in qualitative research is between 15 and 50 participants, with 20 being the average sample size in grounded theory studies (which was also the type of study I was undertaking). Guest, Bunce and Johnson (2005) used data from their own study to conclude that 88% of the codes they developed when analysing the data from 60 qualitative interviews were created by the time 12 interviews had been conducted.

These findings helped me in arguing that my initial sample size was going to be 20. “Given the detailed design of the study, which includes triangulation of the data and methods”, I argued, “I believe that this number will enable me to make valid judgements about the general trends emerging in the data”. I also stated that I am planning to recruit more participants, should the saturation not occur.

I hope that this article will help you in your quest to determine the sample size for your study and give you an idea of how you can go about arguing that it is a well thought-through decision. Do remember, however, that 20 participants may be enough for one study and not enough, or too many, for another. The point of this article was not to argue that 20 participants is a universally right number for a qualitative study, but rather to point to the fact that there is no such universally right number and that you are not the only one struggling to find guidelines regarding the interview sample size, as well as to put forward the concept of saturation as one of possible principles that may guide you in deciding how many participants to recruit for your study.

If you have any questions regarding this topic, comment below or send me a message through my Facebook page.

• UPDATE – see my Facebook page for my response to the question about the relevance of “saturation” for Phenomenological research

References:

Baker, S. & Edwards, R. (eds., 2012). How many qualitative interviews is enough? Expert voices and early career reflections on sampling and cases in qualitative research. National Centre for Research Methods, 1-42.

Guest, G., Bunce, A. & Johnson, L. (2005). How many interviews are enough? An experiment with data saturation and variability. Field Methods, 18 (1), 59-82.

Mason, M. (2010). Sample Size and Saturation in PhD Studies Using Qualitative Interviews. Forum Qualitative Sozialforschung / Forum: Qualitative Social Research, 11 (3).

What is a sample size in qualitative research?

What type of sampling is used in qualitative research?

Is 20 participants enough for qualitative research?