System designers and applied psychologists are increasingly coming to believe that people deal with complex interactive systems by making use of a conceptual model (or mental model) of the system, and that the fostering of this model is a very important consideration for designers (Halasz & Moran, 1983; Norman, 1986; Young, 1981, 1983). A mental model is a cognitive representation of a system that can take inputs from the external world and return predictions of effects for those inputs. Such a model, when appropriate, can be helpful or perhaps even necessary for dealing with the system; but when inappropriate or inadequate, it can lead to misconceptions and errors (Halasz & Moran, 1982; Waren, 1985).

As information systems continue to increase in complexity and power, they will become more difficult to learn and apply. The use of a single metaphor in interface design may not be sufficient for producing an appropriate mental model of any complex system since one metaphor can only represent a subset of a complicated system (Spiro, Feltovich, Coulson, & Anderson, 1989; Spiro, Vispoel, Schmitz, Samarapungavan, & Boerger, 1987). By incorporating multiple metaphors into a interface design, we may be able to create a more powerful representation of a complex system.

Only a few studies have paid attention to the insufficiency of using single metaphors and the power of using multiple metaphors in learning new concepts (Collins & Gentner, 1983, 1987; Gick & Holyoak, 1983; Rumelhart & Norman, 1981; Spiro, et al., 1987; 1989). In the studies of using multiple metaphors, discussions are held at the level of theory construction, with only a few systematic experimental results available. It is crucial that we examine the problems of using a single metaphor as well as investigate the use of multiple metaphors in interface design.

The purpose of this study is to: (a) examine the use of single metaphors and examine the limitations of single metaphors in interface design for complex systems; (b) discover the degree to which the strategy of incorporating multiple metaphors in interface design for complex systems facilitates learning of the complexity of the systems; and (c) investigate the role that training time plays in the learning process.

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