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Good conceptual models are what allow us to predict the effects of our actions. Conceptual models allow the user to predict the machineís output in response to controller inputs (for all of you control systems people out there, this could also be called a transfer function). There is no need to completely understand the physics or chemistry of how a machine works in order to properly use it. All we need to understand is the relationship between our controller inputs and the outputs of the machine. Conceptual models are what allow a person to visualize and mentally simulate the machineís operation. The goal of every control system designer is to develop a controller display which conveys precise information that allows the user to develop a good conceptual model.

Conversely, incorrect or incomplete conceptual models are often what cause us to operate machines incorrectly. A poorly or incorrectly designed controller display will cause the user to develop an incorrect conceptual model., and cause that person to incorrectly visualize and mentally simulate the machineís operation. Unfortunately, examples of poor controller display design are all too common in every day life, and the user often blames himself as in, "Geez, I must be stupid." Poorly designed control displays account for a large number of the infamous "operator error" accidents.

A good example of poor display design which led to a poor conceptual model can be found in Donald Normanís, The Design of Everyday Things, (Doubleday Publishing Group Inc., New York, New York, 1990, page 14). Norman uses the example of the temperature controls of a refrigerator. The author wanted to make the freezer warmer and keep the fresh food compartment a constant temperature.

The poor design of the controls led the author to construct an incorrect conceptual model of the refrigeratorís inner workings and system setup. This, in turn, made it difficult for him to correctly increase the temperature in the freezer compartment while maintaining constant temperature in the fresh food compartment.

It is interesting to note that the refrigerator designer used a good deal of labeling on their controller display. Unfortunately though, the labeling made operation of the refrigerator incomprehensible for situations which were not defined by the labeling. No amount of labeling will fully compensate for a poorly designed display.

Violation of information processing principles and display design principles while designing displays will lead to poor conceptual model development, and result in design-induced error. Humans are exceedingly flexible and can adapt and learn to use poorly designed control display systems. However, the poor designs increase the likelihood that the operator will make errors when operating the controls. If the operation of a controller is not intuitively obvious, the display is probably poorly designed.

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