Yes I have covered this in multiple forms:
Also, complaint driven development:
You don’t need to find all the problems. In fact, you’ll never find all of the problems in anything you test. And it wouldn’t help if you did, because of this fact:
You can find more problems in half a day than you can fix in a month.
You’ll always find more problems than you have the resources to fix, so it’s very important that you focus on fixing the most serious ones first. And three users are very likely to encounter many of the most significant problems related to the tasks that you’re testing.
The above statements by Steve Krug (whose Don’t Make Me Think book I basically worship) are deeply, profoundly true. If you only ever learn one thing about usability, make it this fact.