Is there a big difference between testing 20 users or multiple groups of 5 users?
Nielsen says:
1. First test with 5 users (to catch average of 85% of usability problems)
2. Fix those problems
3. Second test with 5 users (to fixes, tests, and probes much deeper than first
4. Fix those problems
5. Third test with 5 users (to fixes more problems)
6. Then fix no more...
Faulkner says:
"the risk of relying on any one set of 5 users was that nearly half of the identified problems could have been missed; however, each addition of users markedly increased the odds of finding the problems." BUT...Faulkner’s article is based on using 5 users in single usability tests.
Finally, he says:
"...But it sure seems like three groups of 5 users for usability testing still works just fine, and catches most, if not all, web usability problems."
