User Mental Models

From the moviegoer's point-of-view, it is easy to forget the nuance of sprocket holes and light-interrupters while watching an absorbing drama. Many moviegoers, in fact, have little idea how the projector actually works, or how this differs from the way a television works. The viewer imagines that the projector merely throws a picture that moves onto the big screen. This is called the user's mental model, or conceptual model.

People don't need to know all the details of how a complex artifact actually works in order to use it, so they create a cognitive shorthand for explaining it, one that is powerful enough to cover their interactions with it, but which doesn't necessarily reflect its actual inner mechanics. For example, many people imagine that, when they plug their vacuum cleaners and blenders into outlets in the wall, the electricity flows like water from the wall to the appliances through the little black tube of the electrical cord. This mental model is perfectly adequate for using household appliances. The fact that the implementation model of household electricity involves nothing resembling a fluid actually traveling up the cord and that there is a reversal of electrical potential 120 times per second is irrelevant to the user, although the power company needs to know the details.

In the digital world, however, the differences between a user's mental model and the implementation model are often quite distinct. We tend to ignore the fact that our cellular telephone doesn't work like a landline phone; instead, it is actually a radio transceiver that might swap connections between a half-dozen different cellular base antennas in the course of a two-minute call. Knowing this doesn't help us to understand how to use the phone.

The discrepancy between implementation and mental models is particularly stark in the case of software applications, where the complexity of implementation can make it nearly impossible for the user to see the mechanistic connections between his actions and the program's reactions. When we use a computer to digitally edit sound or to create video special effects like morphing, we are bereft of analogy to the mechanical world, so our mental models are necessarily different from the implementation model. Even if the connections were visible, they would remain inscrutable to most people.