User-centered design of any system requires user testing.
The question of how many users should be tested had settled for a
while on about 5-8 users. Today there is a new proposal promoting
the swith of focus from number of users to the number of tasks.
Quoting from HFI (Human Factors International):
“In contrast, their analysis showed a significant positive correlation
between the number of tasks evaluated and the number of problems
uncovered. That is, the more tasks a team included in their testing protocol, the more problems they uncovered.
They conclude that other things being equal (e.g., quality of recruiting), the better predictor of the productivity of usability testing is the number of tasks participants (try to) complete, not the number of participants who try to complete them.”