The Right Number of User Interviews

  • Start with 5
  • Increase by up to 5 more depending on domain complexity
  • Multiply by the number of personas covered
  • Add one to three interviews to work out process kinks if this is new territory or a new team

You’re after saturation.

There’s no single correct number

  • Gaskin, Griffin, Hauser & Co recommend 10–30 interviews that produce 75–150 statements from customers in Voice of the Customer.
  • Daniel Bertaux said “15 is the smallest acceptable sample” in a 1981 paper on sociological research.
  • Greg Guest and co concluded from an ethnographic study that they created 97% of their research codes within 12 interviews and 94% in the first 6.
  • Nielsen Norman says that 5 usability test subjects are all you should target per round.*
  • The 560 qualitative research papers that Mark Mason surveyed in 2010 conducted anywhere from 1–95 interviews, averaging 31 with a large standard deviation.

How to (Roughly) Estimate Your Sample Size

1. Diminishing Returns

2. The size of your domain

3. Your population’s heterogeneity

4. The skill and experience of the interviewer and note-takers

Summing it up

  • Start with 5
  • Increase by up to 5 more depending on domain complexity
  • Multiply by the number of personas covered
  • Add one to three interviews to work out process kinks if this is new territory or a new team

References

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Mitchel Seaman

Mitchel Seaman

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