Mode error

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A mode is a state of the interface which influences the perceived effect of actions. A mode error occurs when a user of something performs an action that is appropriate to a different mode and gets an unexpected and undesired response. A mode error can be quite startling and disorienting as the user copes with the sudden violation of his or her user expectations.

Depending on the design, this undesired response can range all the way from a minor annoyance to a full fledged disaster. The annoyance happens most often in computer software, for example when the caps lock key is accidentally down, or as described in the Excel example in the External links section below. An example of a case that could be a disaster is if a driver is conditioned by a four way stop sign near his or her home to assume that similar intersections will also be four way stops. If it happens to be only two way, the driver could proceed through if he or she sees no other cars, and especially if there is an obstructed view, a car could come though and hit the first car broadside. A more severe example is when a mode error causes an automation surprise to the pilot of an airplane and compromises air safety.

The above example is just one of many that shows why modes should be avoided if at all possible, although there are times when modes are appropriate. If modes must be used, there should at the minimum be clear indicators of the current mode to help prevent mode errors. The Unix text editor vi can be notoriously difficult for beginners precisely because it uses modes, and the default, in early versions anyway, is for mode indication to be turned off. Programmers and other practitioners of interaction design such as Larry Tesler have been using the phrase "Don't Mode Me In" for years as a rally cry to eliminate or reduce modes. [1]

The well-known HCI researcher Donald Norman argues that the best way to avoid mode errors, in addition to clear indications of state, is to construct an accurate mental model of the system for the user which will allow them to predict the mode accurately. This is demonstrated, for example, by some stop signs which show small diagrams describing which of the directions have a stop sign and which don't.

[edit] Additional examples

Users experienced with Unix may find themselves typing ls at a Windows command prompt to list the files in a directory; conversely, experienced Windows users may type dir at a Unix prompt. Here the mode is the command line interpreter being used. This habit is so strong that many users set up aliases to allow both commands to function correctly, and dir is set up to work by default on many Unix systems.