Editor’s note: Peter DePaulo is an independent marketing research consultant and focus group moderator doing business as DePaulo Research Consulting, Montgomeryville, Pa.

In a qualitative research project, how large should the sample be? How many focus group respondents, individual depth interviews (IDIs), or ethnographic observations are needed?

We do have some informal rules of thumb. For example, Maria Krieger (in her white paper, “The Single Group Caveat,” Brain Tree Research & Consulting, 1991) advises that separate focus groups are needed for major segments such as men, women, and age groups, and that two or more groups are needed per segment because any one group may be idiosyncratic. Another guideline is to continue doing groups or IDIs until we seem to have reached a saturation point and are no longer hearing anything new.

Such rules are intuitive and reasonable, but they are not solidly grounded and do not really tell us what an optimal qualitative sample size may be. The approach proposed here gives specific answers based on a firm foundation.

First, the importance of sample size in qualitative research must be understood.

One might suppose that “N” (the number in the sample) simply is not very important in a qualitative project. After all, the effect of increasing N, as we learned in statistics class, is to reduce the sampling error (e.g., the +/- 3 percent variation in opinion polls with N = 1,000) in a quantitative estimate. Qualitative research normally is inappropriate for estimating quantities. So, we lack the old familiar reason for increasing sample size.

Nevertheless, in qualitative work, we do try to discover something. We may be seeking to uncover: the reasons why consumers may or may not be satisfied with a product; the product attributes that may be important to users; possible consumer perceptions of celebrity spokespersons; the various problems t...