Usability testing, as you may well know, involves you watching real users interact with your website or application so you can really see how they interact with it and how you might improve certain areas of it.
Of course, there’s no reason usability testing in software has to be restricted to just websites and applications. In Peter Seibel’s book, Coders at Work, I was interested to read Simon Peyton Jones discussing how Microsoft applies usability testing to API’s:
Well, they also do some interesting work on testing APIs. Steven Clarke and his colleagues at Redmond have made systematic attempts to watch programmers, given a new API, talk through what they’re trying to do. And they get the people who designed the API to sit behind a glass screen and watch them.
And the guys sitting there behind the glass screen say, “No, no, don’t do that! That’s not the right way!” But it’s soundproof. That turns out often to be very instructive. They go and change their API.
from Coders at Work, Chapter 7 - Simon Peyton Jones [p. 253]
If you haven’t yet got a copy of Coders at Work you can buy one from Amazon.