We’re planning our usability testing, i.e. our IEC 62366 - compliant Summative Usability Evaluation. Is there any sort of minimum number of users we need to include?
If you’ve read some standards by now, you can safely assume that they would never state something like this explicitly. There’s a somewhat unwritten rule that the minimum count of users for a usability test is five, based on research by Jakob Nielsen at Sun Microsystems. So that should be your starting point.
But don’t take this too literally and just include five users. If you have the opportunity to acquire more, say 10 or 15, that’s great!
Of course, this heavily depends on how easy you can acquire test users. If your software is geared towards radiologists, it may be difficult and expensive to find any who are available. If it’s however a consumer-facing app, it becomes easy. You could literally ask a bunch of friends.
By the way, Nielsen intention wasn’t to stop usability tests after including five users. Instead, he suggests doing multiple tests (see wikipedia, quote below). The question is whether you have (time) resources to conduct multiple usability test prior to your first certification. Maybe consider doing more after your initial product is certified and you have more money.
It is worth noting that Nielsen does not advocate stopping after a single test with five users; his point is that testing with five users, fixing the problems they uncover, and then testing the revised site with five different users is a better use of limited resources than running a single usability test with 10 users. In practice, the tests are run once or twice per week during the entire development cycle, using three to five test subjects per round, and with the results delivered within 24 hours to the designers. The number of users actually tested over the course of the project can thus easily reach 50 to 100 people. Research shows that user testing conducted by organisations most commonly involves the recruitment of 5-10 participants