Interviews are a commonly used data collection method in qualitative studies, where the goal is to understand or explore a phenomenon. They’re an extremely effective way to gather rich, descriptive data about people’s experiences in a program or exhibition, which is one reason we use them often in our work at RK&A. Figuring out sample size for interviews can sometimes feel trickier than for quantitative methods, like questionnaires because there aren’t tools like sample size calculators to use. However, there are several important questions to consider that can help guide your decision-making (and while you do so, remember that a cornerstone of qualitative research is that it requires a high tolerance for ambiguity and instinct!):
- How much does your population vary? The more homogenous the population, the smaller the sample size. For example, is your population all teachers? Do they all teach the same grade level? If so, you can use a smaller sample size, since the population and related phenomenon are narrow. Generally speaking, if a population is very homogeneous and the phenomenon narrow, aim for a sample size of around 10. If the population is varied or the phenomenon is complex, aim for around 40 to 50. And if you want to compare populations, aim for 25 to 30 per segment. In any case, a sample of more than 100 is generally excessive.
- What is the scope of the question or phenomenon you are exploring? The more narrow the question being explored or phenomena being studied, the smaller your sample size can be. Are you looking at one program, or just one aspect of a program? Or, are you comparing programs or looking at many different aspects of a program?
- At what point will you reach redundancy? This is key for determining sample size for any qualitative data collection method. You want to sample only to the point of saturation—that is, stop sampling when no new information emerges. Another way to think about this is that you stop collecting data when you keep hearing the same things again and again. To be clear, I’m talking about big trends here—while each interview will have its own nuance and the small details might vary from interview to interview, you can stop when the larger trends start to repeat themselves and no new trends arise.
The question of “how many” for qualitative studies might always feel a bit frustrating, since (as illustrated by the questions above) the answer will always be “it depends.” But remember, as the word “qualitative” suggests, it’s less about exact numbers and more about understanding the quality of responses, including the breadth, depth, and range of responses. Each study will vary, but as long as you consider the questions above the next time you are deciding on sample size for qualitative methods, you can be confident you’re approaching the study in a systematic and rigorous way.