The design of a user interface affects task performance times and user opinions of the particular computer program involved. This paper provides details of two of a series of experiments performed during the course of developing a user interface for a Conceptual Bridge Design Expert System (CRDES). The first experiment revealed that for performing a cognitively simple task, users both prefer, and are faster, when using a hierarchical menu structure, in contrast to the simultaneous display of information. The second experiment examined the effects of presenting relevant and irrelevant information on-screen, upon the performance of a more complex task. Presentation of information relevant to the task leads to higher preference ratings and faster performance times than presentation of either irrelevant or no information. Interestingly, presentation of no information results in faster performance times than presentation of irrelevant information does. This finding has important implications for designers of programs for engineering applications, which allow the user to have open several windows on-screen at one time.

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