How many interviews do you need for a PhD thesis?

As a qualitative mentor, I am often asked the question “How do I choose the right sample size?” There is no short answer or hard and fast rule. As with everything, there are nuances here, and it depends a lot on other factors in your degree. The purpose of this blog post is to provide you with some strategies for selecting the appropriate sample for your qualitative study.

When students ask me what example they should have, the first thing I do is consult the literature. In your literature search, you reviewed studies using methods similar to yours. Look at what other people who have done studies like mine have used as examples and start modeling yours on those estimates. This gives you a good starting point to estimate your sample size.

Facilitating focus groups

Facilitating a focus group effectively requires training and practice. Although the facilitator may have passion and enthusiasm for the topic, the purpose of the focus group is to elicit the participants’ perspectives and not to provide a platform for the facilitator to try to influence the participants. An effective facilitator facilitates the participants’ discussion and does not allow one or more participants to dominate the discussion or exert undue influence on other participants. The number of topics that may emerge from a focus group will not be as important as during an individual interview. The results tend to be subjective opinions, especially when the researcher is not neutral in entering the data (Krueger & Casey, 2000; Morgan, 1998).

A similar type of focus group can seem unnatural, in part because the discussion has a formal moderator (Kitzinger & Barbour, 2001). Some participants may not have much experience in group conversations with potential strangers. As a result, dialogue within some focus groups can deepen the expression of experiences and opinions from individual interviews (Kitzinger & Barbour, 2001; Morgan, 1998) and amplify that expression in others.

The moderator has less control in a focus group than in individual interviews. The facilitator, who is often the researcher, may need to repeat or rephrase the original question to resolve inconsistencies in the conversation (Ho, 2006). Facilitator experience can be a factor in the robustness of focus group data (Ho, 2006). Moderators need to be able to encourage all members to participate so that one or more participants do not feel that participating is a waste of time.

Overview: 5 interview mistakes

  1. No clear interview strategy/plan
  2. No good interview techniques/skills
  3. appropriate equipment

  4. Without a basic risk management plan
  5. Not keeping the “golden thread” in mind

The first common mistake we make The most important thing is to close starting the interview without first developing a clear conversation strategy or action plan. Of course, you’ll want to connect with your interviewees, but a lack of planning can lead to messy details and inconsistencies between interviews.


Baker, S. & Edwards, R. (eds., 2012). How many qualitative interviews are enough? Expert voices and early career reflections on case sampling and qualitative research. National Center for Research Methods, 1-42.

Gast G, Bunce A & Johnson L (2005). How many calls is enough? Experience with data saturation and data variability. Field Methods, 18(1), 59-82.

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