Discuss Why you shouldn't ask 'Why?' in a usability test

Sometimes it's not just a case of not asking "why" but also ignoring what people say unprompted, or just because you asked them to "think aloud". I've had people offer excuses for their inability to use something without any questioning. Sometimes it gives you some insight, sometimes it doesn't, and it's hard to tell which it is! But the important thing to note is that they were having difficulty.

This assumption virtually guarantees that negative impacts will be over- estimated, while positive impacts will be underestimated.