Kathleen Straub has written a nice overview of the difference between an Expert Review and User Testing called “Cleaning Up For The Housekeeper”.
The whole article is good, but here’s a key point she makes:
“Expert Review examines details of human computer interaction guided by basic research about how humans interpret, understand and interact with objects in the world. As such, Heuristic Review exploits our generic understanding of human cognition to identify design/presentation details that may facilitate or impede a user’s progress within a task. These include issues such as affordances (How obvious the right next-thing-to-do is.), consistency and the effectiveness of layout and color to guide the user experience.
Usability testing identifies gaps between the site model and representative user conceptual use model in the specific context of use. Meaningful usability testing means observing representative users doing things on the site. Users bring unique domain knowledge and experience to their user experience. Designers — even experts — don’t have the same perspective.”
The title comes from the distinction between “straightening” and “cleaning”. You don’t hire a “straightening lady” so you need to straighten first so she can do her job. In the same way, it makes sense to do an expert review first (to “straighten”) and then do usability testing (to “clean”).