Exactly - users aren't homogeneous. Neither are UIs - which are often meant to be used by different types of user group. You'll notice the author massively cav

Exactly - users aren't homogeneous. Neither are UIs - which are often meant to be used by different types of user group. You'll notice the author massively caveats his article near the end, stating "The best strategy is to bring in some set of users, find the problems they have, fix those problems, then bring in another set of users as part of an iterative design and test strategy. In the end, although you're never testing more than 5 users at a time, in total you might test 15 or 20 users."