In a qualitative research project, how large should the sample be? How many focus groups, individual depth interviews (IDIs), or online bulletin board focus groups are needed? One suggested rule is to make sure you do more than one group on a topic because any one group may be idiosyncratic. Another guideline is to continue doing groups or IDIs until we have reached a saturation point and are no longer hearing anything new. These rules are intuitive and reasonable, but they are not solidly grounded and do not really give us an optimal qualitative sample size. The approach proposed here gives some specific answers. First, the importance of sample size in qualitative research must be understood.

Size Does Matter, Even for a Qualitative Sample 

In qualitative work, we are trying to discover something. We may be seeking to uncover the reasons why consumers are or are not satisfied with a product; the product attributes that are important to users; possible consumer perceptions of celebrity spokespersons; the various problems that consumers experience with our brand; or other kinds of insights. It is up to a subsequent quantitative study to estimate, with statistical precision, the importance or prevalence of each perception. The key point is this: Our qualitative sample must be big enough to ensure that we are likely to hear most or all of the perceptions that might be important.

Discovery Failure Can Be Serious 

What might go wrong if a qualitative project fails to uncover an actionable perception (or attribute, opinion, need, experience, etc.)? Here are some possibilities: 

A source of dissatisfaction is not discovered—and not corrected. In highly competitive industries, even a small incidence of dissatisfaction could dent the bottom line. In the qualitative testing of an advertisement, a copy point that offends a small but vocal subgroup of the market is not discovered until a public relations fiasco erupts. When qualitative procedures are used to pretest a quantitative questionnaire, an undiscovered ambiguity in the wording of a question may mean that some of the subsequent quantitative respondents give invalid responses. Thus, qualitative discovery failure eventually can result in quantitative estimation error due to respondent mis comprehension.

Questions

1. Is sample size important in qualitative research? Why do you say that?

2. What is a reasonable target sample size for qualitative studies? What is that sample size based on?