How many customer interviews are enough?

Counts of customer interviews seem to have become a bit of a vanity metric of late. A shorthand for product or decision quality, as if one automatically implies the other.

I appreciate your sacrifice at the temple of customer research, but I worry that you may have wasted your time.

Working out the right number of interviews, wireframe tests or customers in the alpha phase of your project is quite similar to an optimal stopping problem. You’re trying to work out how much learning you should do before leaping to an answer. Optimal stopping problems have a solution, and that solution gives us a useful heuristic for this type of question: you can stop searching and settle on an answer after you have reviewed 37% of the available options. We can use this rule to help us form some guidelines for our product work.

So if you’re spending 2 weeks in product discovery, then plan for ~37% of that time being spent on interviews - get as many as you can into your calendar for 3-4 days. Probably 5<x<10 is enough to get what you need. If it takes 3 days to design your mockup, then a day with users doing some testing is enough (2<x<4 perhaps).

Of course, those interviews don’t give us perfect insight, so adjust up or down based on the quality of the insights and whether you feel that you are hitting the bar to the extent that you are confident about where you’re headed next.

I also don’t mean you should stop talking to customers, I just mean that you shouldn’t wait on moving forwards until you’ve hit an arbitrary but impressive sounding number. Customer learning should be continuous, but great execution requires us to shift through gears quickly, validating what we’re building any way we can as we go.

If we want to get skilled at building things fast, we need to cut waste where we can. This is one of them.