6. Customer Interviews That Do Not Mislead You: How Many to Run
After this lesson you can
- Design interview questions about past behavior, workarounds and money and time already spent
- Distinguish stated preference from revealed preference
- Plan about 12 to 30 interviews per distinct segment and explain the saturation studies behind that range
- Explain why interview counts find needs but do not prove demand
- Talk about problems without explaining an invention, and apply the ethics checkpoint before collecting data
- Analyze notes by theme and compare results with thresholds set in advance
Video coming soon
Videos are planned to publish from December 2026.
A lesson on the bottom rung of the evidence ladder: problem interviews. It explains why polite, hypothetical answers mislead, what saturation research says about interview numbers, and how to keep interviews problem-focused and ethically checked.
Questions and answers
Questions a learner would ask about this lesson. Think through your own answer before you open each one.
- Three people told me they would definitely use my product. Isn't that validation?
Show answer
Not on its own. People are polite to someone with an idea, and hypothetical questions invite hopeful guesses. "I would definitely use it" is stated preference, which costs nothing to give. Revealed preference, what people's past actions show they value through time, money or effort spent on workarounds, is stronger evidence.
- What makes a good problem-interview question?
Show answer
It asks about the last time the problem happened, what the person does about it now, what they have already spent, and who else was involved. Follow-ups such as "Can you walk me through that?" and "Can you show me?" get specifics. A problem interview is not a pitch, so you do not present a solution.
- How many interviews should I plan?
Show answer
The lesson turns three studies into a planning range of about 12 to 30 interviews for each distinct, homogeneous segment, nearer 12 for spotting themes and nearer 30 for depth. Write your rationale before you start. The studies came from specific, fairly similar groups, so the numbers are guides, not rules.
- Do enough interviews prove demand?
Show answer
No. The saturation studies measured how quickly themes and needs were found; none measured whether anyone bought anything. Interview counts help you find needs in a segment, and demand still needs behavioral testing and commitments, higher rungs on the evidence ladder.
- What is the ethics checkpoint?
Show answer
Before collecting data from people, ask: could any of this appear in a thesis, paper or talk? If yes or maybe, contact the research ethics office before collecting. If no, write down your purpose anyway, and ask again if plans change, because purposes drift.
Practice quiz
- Question 1Hennink and colleagues found meaning saturation required about:
- Question 2Which question best follows the past-behavior principle?
- Question 3Founder B plans interviews with nonprofit directors and with graduate researchers. For planning, these should be treated as:
- Question 4Founder C may present interview findings at a conference later. Under the ethics checkpoint, they should:
- Question 5Twenty-five interviews with clinic managers produced consistent problem themes. What can Founder A conclude?
- Question 6In the Hennink study, code saturation came at 9 interviews. What does code saturation mean?
- Question 7Founder A plans 15 interviews each with clinic managers and with front-desk staff. How many interviews is that in total?
- Question 8In Griffin and Hauser (1993), how many interviews identified over 90 percent of the needs found in 30?
- Question 9Founder D wants to interview utility operators without raising disclosure risk. Which approach does the lesson describe as lower-risk practice?
- Question 10Collecting only the personal data needed for a stated purpose, and no more, is called:
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