Sample size for Qualitative Interviews

Through the looking Glass

This is one of life’s great mysteries and one that has troubled many a qualitative researcher. How many interviews are enough before i have enough data? What should be my sample size for Qualitative Interviews? During my research this was a issue I really struggled with and found that justifying a sample size was not an easy task and required me to juggle multiple variables.

This short post covers some of the key considerations researchers need to consider and the issue of data saturation and the sample size for Qualitative Interviews.

Justification of Sample

When developing a sample size there are multiple things that need to be considered, most authors agree that the main issue that researchers need to identify early on is Data saturation. The researcher in determining if they have arrived at data saturation should ask themselves:

• Have I hit the peak of my data?
• Have I collected so much data that no new phenomena will now emerge?
• Have I missed anything?

The flipside to data saturation is data overload, I once met a PHD student who administered over 35 in-depth interviews which each lasted over an 1 hour and 30 mins long. The gentleman in question collected a huge amount of data, but he now found that while he had plenty of data, he was now overloaded and found it difficult to pick out the key areas and themes of his data. He struggled to break down the key aspects.

Ritchie et al. (2003), suggests that within qualitative research the sample size is usually small primarily because phenomena only need to appear once to be part of the analytical map. After a while there is a point of diminishing return when increasing the sample size no longer contributes to new evidence. Diminishing return is a problem that occurs depending on the type of data being collected, Crabtree & DiCicco-Bloom (2006) refer to this process of diminishing return as data saturation and a signal that the data collection process is now complete or near completion.

In the view of Guest et al. (2006) data saturation can occur within the first twelve interviews and after that very few new phenomena are likely to emerge. In the view of Gonzalez (2009) when undertaking research that is reliant on a phenomenological approach, the sample size is usually driven by the need to uncover all the main variants within the approach, he suggests that within conditions such as this, small survey samples of less than twenty are common. Finally the view of Creswell (2011) in relation to sample size is that normally within qualitative research it is typical “to study a few individuals or a few cases” (pg. 209).

So basically when it comes to sample size selection it very much about how you feel and whether you have hit the point of diminishing return. Personally I feel that within a qualitative interview based on a phenomenological approach it is not uncommon to see samples sizes of between 15-25. 

If you would like to discuss this in more detail or you would like to add anything please leave a comment below.

References

Legard, R., Keegan, J., & Ward, K. (2003). In-depth interviews. In J. Ritchie & J. Lewis (Eds.), Qualitative research practice: A guide for social science students and researchers (pp. 138–169).

B. F., & DiCicco-Bloom, B. (2006). The qualitative research interview. Medical Education, 40(4), 314–318.

Guest, G., Bunce, A., & Johnson, L. (2006). How Many Interviews Are Enough?: An Experiment with Data Saturation and Variability. Field Methods, 18(1), 59–82. doi:10.1177/1525822X05279903

Gonzalez, C. (2009). Conceptions of, and Approaches to, Teaching Online: A Study of Lecturers Teaching Postgraduate Distance Courses. Higher Education, 57(3), 299–314. doi:10.2307/40269124

Creswell, J. W. (2011). Educational Research: Planning, Conducting, and Evaluating Quantitative and Qualitative Research. (Fourth.). Pearson.