Designing a Quality In-depth Interview Study: How Many Interviews Are Enough?
September 12, 2012
Here is a topic you don’t read much about, particularly in the marketing research community: What is the optimal number of in-depth interviews to complete in an IDI study? The appropriate number of interviews to conduct for a face-to-face IDI study needs to be considered at two key moments of time in the research process – the initial research design phase and the phase of field execution. At the initial design stage, the number of IDIs is dictated by four considerations: 1) the breadth, depth, and nature of the research topic or issue; 2) the hetero- or homogeneity of the population of interest; 3) the level of analysis and interpretation required to meet research objectives; and 4) practical parameters such as the availability and access to interviewees, travel and other logistics associated with conducting face-to-face interviews, as well as the budget or financial resources. These four factors present the researcher with the difficult task of balancing the specific realities of the research components while estimating the optimal number of interviews to conduct. Although the number of required interviews tends to move in direct step with the level of diversity and complexity in the research design, there is little guidance in sample size for the researcher at the planning stage.
The other key moment in time when the researcher considers the adequacy of the sample size is during the field phase when interviews are actually being conducted. This has been the most widely discussed point in time by many researchers because it is then, when in the field, that the optimal number of interviews is determined. Specifically, researchers utilizing grounded theory rely on the notion of “saturation” or the point in time when responses no longer reveal ‘fresh insights’. On this basis, the researcher deems that a sufficient number of interviews have been conducted when no new themes or stark variations in interviewees’ responses are coming to light. There are few guidelines for determining number of interviews by way of saturation, and some have questioned its value given the lack of transparency.
Another, more quality approach to the question of how many face-to-face IDIs to conduct considers the design-phase as well as results in the field but goes further. It is not good enough to simply evaluate interview completions in the field based on the point of saturation. While it is important to determine the degree to which interviews are or are not reaping new meaningful information (see the fourth question, below), there are many other quality concerns that need to be resolved. To assess the number of face-to-face IDIs at the field stage, the researcher needs to more broadly review the quality of the interview completions based on the answers to these eight questions:
- Did every IDI cover every question or issue important to the research?
- Did all interviewees provide clear, unambiguous answers to key questions or issues?
- Does the data answer the research objective?
- To what extent are new ideas, themes, or information emerging from these interviews?
- Can the researcher identify the sources of variations and contradictions in the data?
- Does the data confirm or deny what is already known about the subject matter?
- Does the data tell a story? Does it make sense and does it describe the phenomenon or other subject of the study?
- Are new, unexplored segments or avenues for further research emerging from the data?
From there, the researcher can determine whether additional interviews are justified.