You should also look for opportunities to refine, expand, and enhance your index entries. Keep in mind that usability is like beauty. It is in the eye of the beholder. What is useful to you at this particular moment may not be useful to you next week. Also, information that is useful to you may not be useful to a coworker.
Quality is a little easier to define.
A quality index is one that is accurate and complete, free of errors, and consistent in style and terminology. Unfortunately, as easy as it is to define quality, achieving it is not always accomplished as easily. We are, after all, human. We all make mistakes. Our fingers occasionally land on the wrong keys, resulting in embarrassing typos.