Inadvertent errors

A multi-feature system providing various operational options is typically error-prone. For example, suppose that the user of a MS-Work wishes to type in the letter A, but instead of pressing the Shift key, the user inadvertently presses the Ctrl key. In MS-Word, Ctrl-A is a shortcut key combination for ‘Select All’. The next key press erases the selected text, because MS-Word has a feature of ‘Typing replaces selection’. Inexperienced users often respond hysterically to the screen change, by saving the changes. Only after becoming experienced they learn that they can undo the last actions and thereof retrieve the erased text. A well-known enabler of user slips is the text editor Xemacs by GNU. This editor uses almost all possible key combinations as shortcuts for its commands, making it extremely error-prone.

Multi-control commands

Developers of computer software love to provide users with alternative means to issue the same commands; pull-down menu, popup menu, toolbar button and shortcut keys. In multi-feature programs, each of the means is over-populated, and the users eventually waste time trying to find the command they need. For example, suppose that a computer program has 50 commands, and that each command can be executed by any of these features. The user needs to learn 4x50 = 200 controls instead of 50, and each of the controls is more error-prone than with any simple solution having only 50 controls.

Control design

Controls should be selected to fit the position of the command in the operational procedure. Triggers of operational procedures should be included in a main popup menu. Context sensitive actions should be activated by popup menus, and should not appear on pull-down menus. Shortcut keys and toolbar buttons should be used sparsely, for those features that are used extensively. Shortcut keys should be reserved for actions within keyboard-dominated tasks, such as data entry, while toolbar buttons should be reserved for actions within mouse-dominated tasks, such as cut and paste. 

Direct mapping

A main source of the user slips is the need for extra caution before data entry or command execution. The operational procedure should enable carefree, seamless interaction. This is possible only if the mapping from intention to action is direct, namely, if the actuated command is independent of any condition. It should be noted that the principle of direct mapping is actually a rephrasing of the rule for state independence.