Usability for Nerds/Track user behavior
You may get additional information from tracking actual user behavior. On a web site you can simply use the server logs. Several programs are available for analyzing server logs. On other systems you may set up a system for logging everything that users do. These logfiles can then be analyzed statistically.
If the statistics reveal that many users are making the same errors or have the same problems, then you have found a usability problem that should be solved.
The statistics may also be useful for dividing users into categories. If those users that use function A also use function B, while other users use only function C and D, then you can use this information in the design of your menu system so that A and B are in the same submenu, while C and D are in another submenu.
If your system has a search facility, then you should make a log of search misses. This will reveal if important topics are missing in your system, if users use synonyms that you haven't predicted, or spell words in peculiar ways. If several users have searched in vain for the same word then that item should be added to the system.
The user data in your logs should probably be anonymized to avoid concerns about violation of privacy.