The Design of Everyday Things
Donald A. Norman
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size and weight were perilously similar.”4 The confusions probably occurred because the users of coins formed representations in their memory systems that were sufficiently precise only to distinguish among the coins that they actually had to use. It is a general property of memory that we store only partial descriptions of the things to be remembered, descriptions that are sufficiently precise to work at the time something is learned, but that may not work later on, when new experiences
should go without saying, but in fact, it is necessary to say it. There is a simple, well-known solution to the grouping problem: set the switches for one set of functions apart from the switches that control other functions. Another solution is to use different types of switches. The solutions can be combined. To solve the problem with the airplane flap and landing gear switches, separate the switches and don’t line them up in a row. Also use shape coding: a tire-shaped switch can control
likely where the equipment does not make the mode visible, so the user is expected to remember what mode has been established, sometimes for many hours. Mode errors are common with digital watches and computer systems (especially text editors). Several accidents in commercial aviation can be attributed to mode errors, especially in the use of the automatic pilots (which have a large number of complex modes). DETECTING SLIPS Although slips are relatively easy to detect because there is
concerns. The design is almost certainly altered as it leaves the designers and proceeds through manufacturing and marketing. All participants are well intentioned, and their particular concerns are legitimate. The factors should all be considered simultaneously, however, and not subject to the accidents of time sequence or the realities of corporate rank and clout. One designer wrote me this about his problems: “Most designers live in a world where the gulf of evaluation is infinite. True,
just didn’t seem dangerous, that’s all. The same story is repeated over and over again. Underwater divers focus so much on struggling to the surface that they fail to release the lead weights (on a special easy-to-release belt) that are keeping them underwater. People who are fleeing a fire push hard against a door, harder and harder, failing to recognize that the door opens by pulling. Someone is trapped behind a door, pushing against the left side when it opens from the right.