Conceptual model

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The actions and objects contained by a system an user interacts with.

References

In the software world, the first known use of this metaphor is by David Liddle’s team during the Xerox Star development. When those people left Xerox to work in other places the term was expanded to industry: Bill Verplank popularize it within the interaction design community.

Extract of an interview with David Liddle:

ASD: Given the novelty of the ideas, the development must have called for going beyond the standard process. How did you envision your task?

DEL: We tackled it in a very different way from most software development groups. Before we designed the Star, I commissioned a study that produced a document on the methodology for the design of user interface. We borrowed additional people from PARC to help us work on it. It laid out an approach—a thorough approach—to doing software design. That document and that methodology were the basis for the whole Star design process. It began with task analysis, looking at a fairly wide range of users. Next came the job of developing scenarios for uses of the imagined product based on the task. Then, it proposed a model for a graphical user interface, carefully distinguishing three aspects: the display of information, the control or command mechanisms, and the user’s conceptual model.

The first component—information display—deals literally with what appears on the screen. It encompasses all those relatively minor aspects, like what window borders and buttons should look like, what fonts are used and where, what icon shapes are used, and so on. This component is important, but is not the crucial concern from the standpoint of usability. Information display is the least important of the three separate components.

The second component is the control mechanism—the machinery used to invoke commands. It is extremely important that command invocation be designed consistently across different applications. In terms of usability, this component is much more important than information display.

The most important component to design properly is the third, the user’s conceptual model. Everything else should be subordinated to making that model clear, obvious, and substantial. That is almost exactly the opposite of how most software is designed.

ASD: Just what do you mean by a user’s conceptual model?

DEL: This model represents what the user is likely to think, and how the user is likely to respond. For example, one of the events that changed civilization in the past decade was Dan Bricklin’s choice of the spreadsheet metaphor and its underlying conceptual model (see Profile 11). Neither the information display (limited by the machines that were available, and by how much space his program, VisiCalc, required), nor the command invocation were great. However, that conceptual model was exceptionally durable. The desktop metaphor for managing objects—for filing and printing and mailing and the like—is another widely applicable metaphor.