Design of Everyday Things - Norman, Don [101]
7.1 Three Aspects of Mental Models. The design model, the user’s model, and the system image. (From Norman, 1986.)
All three aspects are important. The user’s model is essential, of course, for that determines what is understood. In turn, it is up to the designer to start with a design model that is functional, learnable, and usable. The designer must ensure that the system reveals the appropriate system image. Only then can the user acquire the proper user’s model and find support for the translation of intentions into actions and system state into interpretations. Remember, the user acquires all knowledge of the system from that system image.
THE ROLE OF MANUALS
The system image includes instruction manuals and documentation.
Manuals tend to be less helpful than they should be. They are often written hastily, after the product is designed, under severe time pressures and with insufficient resources, and by people who are overworked and underappreciated. In the best of worlds, the manuals would be written first, then the design would follow the manual. While the product was being designed, potential users could simultaneously test the manuals and mock-ups of the system, giving important design feedback about both.
Alas, even the best manuals cannot be counted on; many users do not read them. Obviously it is wrong to expect to operate complex devices without instruction of some sort, but the designers of complex devices have to deal with human nature as it is.
SIMPLIFY THE STRUCTURE OF TASKS
Tasks should be simple in structure, minimizing the amount of planning or problem solving they require. Unnecessarily complex tasks can be restructured, usually by using technological innovations.
Here is where the designer must pay attention to the psychology of the person, to the limits on how much a person can hold in memory at one time, to the limits on how many active thoughts can be pursued at once. These are the limitations of short-term and long-term memory and of attention. The limitations of short-term memory (STM) are such that a person should not be required to remember more than about five unrelated items at one time. If necessary, the system should provide technological assistance for any temporary memory requirements. The limitations of long-term memory (LTM) mean that information is better and more easily acquired if it makes sense, if it can be integrated into some conceptual framework. Moreover, retrieval from LTM is apt to be slow and to contain errors. Here is where information in the world is important, to remind us of what can be done and how to do it. Limitations on attention are also severe; the system should help by minimizing interruption, by providing aids to allow for recovery of the exact status of the operations that were interrupted.
A major role of new technology should be to make tasks simpler. A task can be restructured through technology, or technology might provide aids to reduce the mental load. Technological aids can show the alternative courses of action; help evaluate implications; and portray outcomes in a more complete, more easily interpretable manner. These aids can make the mappings more visible or, better, make the mappings more natural. Four major technological approaches can be followed:
• Keep the task much the same, but provide mental aids.
• Use technology to make visible what would otherwise be invisible, thus improving feedback and the ability to keep control.
• Automate, but keep the task much the same.
• Change the nature of the task.
Let us look separately at each of these possibilities.
KEEP THE TASK MUCH THE SAME, BUT PROVIDE MENTAL AIDS
Don’t underestimate the power or