When I say “usability test,” you may imagine something that looks like a psych experiment: The “Subject” is in one room, with a stack of task cards and may even have biometric sensors attached. The “Researcher” is in another room, madly logging data and giving instruction over an intercom as the voice of god.
That image of a usability test is what I’d call “formal usability testing,” and is probably going to be summative and validating. It’s a way to verify whether the design does what you want it to do and works the way you want it to work.
This is often the kind of test done toward the end of a design cycle. What I’m interested in—and I think most of you are interested in—is how to explore and evaluate in the early and middle stages of a design.