There are four principles for usability:

 

-          Provide a good conceptual model.

 

 

The design model is the designer’s conceptual model. The user’s model is the mental model developed through interaction with the system. The system image results from the physical structure that had been built. The designer expects the user’s model to be identical to the design model. But the designer doesn’t talk directly with the user – all communication takes place through the system image. If the system image does not make the design model clear and consistent, then the user will end up with the wrong mental model.

 

-          Make things visible.

 

It is important that in the design the possibilities are visible. When for instance, controls have multiple functions, but not all functions are clear, the system is not understandable; its capabilities aren’t apparent. In fact, the relationships among the user’s intentions, the required actions, and the results are completely arbitrary.

 

-          The principle of mapping

 

Mapping is a technical term meaning the relationship between two things, in this case between the controls and their movement and the results in the world. Consider the mapping relationships involved in steering a car. To turn a car to the right, one turns the steering wheel clockwise. The user must identify two mappings here: one of the controls on the dashboard controls affects the steering, and the steering wheel must be turned in one of two directions. Both are somewhat arbitrary.  But the wheel and the clockwise directions are natural choices: visible, closely related to the desired outcome, and providing immediate feedback. The mapping is easily learned and always remembered.

 

 

-          The principle of feedback

 

Feedback – sending back to the user information about what actions has actually been done, what result has been accomplished – is a well-known concept in the science of control and information theory. Imagine trying to talk to someone when you cannot even hear your own voice: there would be no feedback.