In qualitative research, the qualitative sample size is the number of subjects in a study. Qualitative research depends on rich and vivid descriptions of people and their words and actions in the environment being studied. A qualitative sample size is usually relatively small, ranging anywhere from one to 15 people on average. This is different from quantitative research, which is mathematically and statistically based research that relies on much larger samples, sometimes as large as 1,000 subjects or more. A quantitative study, for example, might include comprehensive statistics about the survey responses of 1,000 subjects who were asked about their views on religion, while a qualitative study on the same subject might include interviews with only three people.
The main reason why a qualitative sample size can be very small is that a qualitative research study depends on being able to glean rich and detailed data from its subjects. While it may be appropriate, but not necessary, to include some direct quotes and anecdotes from the subjects in a quantitative study, these kinds of details are absolutely essential to qualitative research studies. A qualitative research study, for example, might focus on having in-depth interviews with 10 students and two teachers about the reasons why students drop out of a school with a high dropout rate.
Another example of a qualitative study might be more long-term and involve following three university students in a teacher education program during their senior year of college and for the first five years of their teaching careers to see to what degree they implemented the education methods they were taught in their undergraduate education courses. A study such as this would require both detailed observations of the teachers’ behavior in the classroom and in-depth interviews with them. The result would be a limited, rather than generalized, conclusion about their use of teaching methods learned in college and their reasons for using or not using them.
There are instances in this type of research when a qualitative sample size of one would be appropriate. A psychology case study, for example, might focus on one client diagnosed with intermittent explosive disorder with whom a clinician worked over a period of time. In this case study, the psychologist could try to help the client improve his or her outbursts of anger and resume a more normal life by experimentally using cognitive strategies and behavioral management techniques. The psychologist working with the client also could act as a qualitative researcher by documenting the interventions and their success, or lack thereof, with this one person. Observations of the one client and interviews with him or her — and possibly with people closely involved in the patient's life — could provide the rich and detailed data required of qualitative studies.