13.6-8 Functionally Related Adjustments

If a single control is used to affect multiple variables, then the user interface should be designed to prevent mode errors.

Additional Information:
Mode errors occur when the user performs an action that is appropriate for one mode when a different mode is in effect. Four design strategies for preventing mode errors are eliminating modes, making modes distinct, providing different inputs for different modes, and coordinating inputs across modes. Eliminating modes prevents mode errors by eliminating the conditions under which they occur (i.e., if there are no modes there can be no mode errors). Making modes distinct deals with the problem through feedback. By saliently indicating the currently active mode, operators are more likely to be aware of it and less likely to provide an incompatible input. Providing different inputs for different modes addresses the problem by ensuring that the same input is not valid in more than one mode. Thus, if the operator provides an input while in the wrong mode, the system will not accept it. Coordinating inputs across modes ensures that a command producing a benign effect in one mode does not produce a severely negative effect in another mode.