Free tool
Usability test participants calculator
How many people do you need to test with? Set how likely a typical user is to hit a given problem and how much of the problems you want to find, and the calculator does the maths per user group.
How it works
The calculator uses the standard model from usability research: the proportion of problems found after *n* participants is 1 − (1 − p)ⁿ, where *p* is the chance that one participant hits a given problem. With the classic value of 31%, five participants find about 85% of problems, which is why five is the usual number for a quick round.
When to change the defaults
- Lower p (10 to 20%) for complex, specialist or infrequent tasks, where each person only sees part of the service.
- Higher target when the cost of a missed problem is high, for example a benefits application or a medical device.
- More groups when audiences behave differently: claimants and case workers, consumers and agents, new and returning users. Each group needs its own sample.
What it does not tell you
It estimates how many problems you will *see*, not how important they are. That is what the researcher in the room is for.
Want us to run the study?
We recruit, moderate, analyse and report, in person or remotely.