How many customer interviews do you need? Fewer than you think, then check

How many customer interviews do you need? For one focused question, plan 4 to 8, then read your notes before booking more. GOV.UK's service research guidance suggests usually 4 to 8 participants per round for methods including in-depth interviews. That is a starting budget for one question and one audience, not proof of demand.

Count conversations, not invitations: eight invitations sent is not eight conversations. Leave room in your budget and calendar to adjust.

1. Read your notes before you book another call

Ask what you can now explain and which open question could change what you do next. The interview analysis guide shows how to check a conclusion against the original notes. (Rereading is the cheapest interview you will ever run.)

Repeated answers help, but check whose they are. Several people doing the same job can describe a process well and still leave the buyer's part unexplained.

2. Stop when you can make the decision

A round number is not a reason to keep going. Stop when you can make the decision you started with, or when the remaining question needs a different kind of test.

3. Let the open question pick the next step

Here is a made-up example. You have interviewed dispatchers at repair companies, and several describe waiting for the owner's approval before changing a technician's assignment. What next depends on what you still need to learn:

• Why was approval needed? Check whether your notes explain it. If not, talk to an owner who recently made that call about that actual reassignment.
• Can a dispatcher use your new reassignment screen? Give them a realistic task and watch them try. Another description of the old process won't tell you.
• Will a company commit time or money to the tool? Offer something concrete, such as a pilot with a set price and workload, and see whether the company commits. Agreeing the current process is frustrating is not buying.

4. "Five users" is a usability rule

The familiar "five users" recommendation is about finding usability problems, fixing the design and testing again. It does not mean five customer interviews prove demand.

Name the question before you pay for the call

Write down the question the next paid call should answer, then choose someone with the relevant experience. In Instant Expert, check the displayed request price and put that question in the invitation. The person decides whether to accept, so plan your interview budget with some slack.