Throughout this book, the merits of an interface are assessed from a usability perspective; this chapter discusses how these assessments are done. It is surprisingly difficult to design a usable new search interface, and perhaps even harder to convincingly assess its usability (Draper and Dunlop 1997). An evaluator must take into account differences in designs, tasks, participant motivation, and knowledge, all of which can vary the outcome of a study. Furthermore, as mentioned in Chapter 1, small details in the design of the interface can have a strong effect on a participant's subjective reaction to or objective success with the interface. For instance, problematic placement of controls or unintuitive text on a hyperlink can prevent a participant from succeeding in a task. Differences in font contrast and spacing can unconsciously affect a participant's subjective response to a design.

What should be measured when assessing a search interface? Traditional information retrieval research focuses on evaluating the proportion of relevant documents retrieved in response to a query. In evaluating search user interfaces, this kind of measure can also be used, but is just one component within broader usability measures. Recall from Chapter 1 that usable interfaces are defined in terms of learnability, efficiency, memorability, error reduction, and user satisfaction (Nielsen 2003b; Shneiderman and Plaisant 2004).