Reading Usable Help
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Gordon R. Meyer
User testing can be a waste of time
In my experience, user testing can be helpful if you're trying to break new ground and just want a quick gut-check on the interaction design. But the desire to run "extensive user testing" as a way to "prove" something is really not at all productive or conclusive. I think you can find all the major problems in a single day, with a handful of test subjects, and that ultimately you have to trust your professional instincts anyway. This seems to jive with the opinion offered in The Myth of Usability Testing.