Emily Namey, R&E Search for Evidence http://researchforevidence.fhi360.org/author/enamey [2]
“The first two posts in this series describe commonly used research sampling strategies [3] and provide some guidance on how to choose from this range of sampling methods [4]. Here we delve further into the sampling world and address sample sizes for qualitative research and evaluation projects. Specifically, we address the often-asked question: How many in-depth interviews/focus groups do I need to conduct for my study?
Within the qualitative literature (and community of practice), the concept of “saturation” – the point when incoming data produce little or no new information – is the well-accepted standard by which sample sizes for qualitative inquiry are determined (Guest et al. 2006 [5]; Guest and MacQueen 2008 [6]). There’s just one small problem with this: saturation, by definition, can be determined only during or after data analysis. And most of us need to justify our sample sizes (to funders, ethics committees, etc.) before collecting data!
Until relatively recently, researchers and evaluators had to rely on rules of thumb or their personal experiences to estimate how many qualitative data collection events they needed for a study; empirical data to support these sample sizes were virtually non-existent. This began to change a little over a decade ago. Morgan and colleagues (2002) [7] decided to plot (and publish!) the number of new concepts identified in successive interviews across four datasets. They found that nearly no new concepts were found after 20 interviews. Extrapolating from their data, we see that the first five to six in-depth interviews produced the majority of new data, and approximately 80% to 92% of concepts were identified within the first 10 interviews.
Emily’s blog continues here http://researchforevidence.fhi360.org/riddle-me-this-how-many-interviews-or-focus-groups-are-enough [8]