Design of Everyday Things - Norman, Don [59]
CHAPTER FIVE
TO ERR IS HUMAN
“LONDON—An inexperienced computer-operator pressed the wrong key on a terminal in early December, causing chaos at the London Stock Exchange. The error at stockbrokers Greenwell Montagu led to systems staff working through the night in an attempt to cure the problem. ”1
People make errors routinely. Hardly a minute of a normal conversation can go by without a stumble, a repetition, a phrase stopped midway through to be discarded or redone. Human language provides special mechanisms that make corrections so automatic that the participants hardly take notice; indeed, they may be surprised when errors are pointed out. Artificial devices do not have the same tolerance. Push the wrong button, and chaos may result.
Errors come in several forms. Two fundamental categories are slips and mistakes. Slips result from automatic behavior, when subconscious actions that are intended to satisfy our goals get waylaid en route. Mistakes result from conscious deliberations. The same processes that make us creative and insightful by allowing us to see relationships between apparently unrelated things, that let us leap to correct conclusions on the basis of partial or even faulty evidence, also lead to error.
Our ability to generalize from small amounts of information helps tremendously in new situations; but sometimes we generalize too rapidly, classifying a new situation as similar to an old one when, in fact, there are significant discrepancies. False generalizations can be hard to discover, let alone eliminate.
The differences between slips and mistakes are readily apparent in the analysis of the seven stages of action. Form an appropriate goal but mess up in the performance, and you’ve made a slip. Slips are almost always small things: a misplaced action, the wrong thing moved, a desired action undone. Moreover, they are relatively easy to discover by simple observation and monitoring. Form the wrong goal, and you’ve made a mistake. Mistakes can be major events, and they are difficult or even impossible to detect—after all, the action performed is appropriate for the goal.
Slips
A colleague reported that he went to his car to drive to work. As he drove away, he realized that he had forgotten his briefcase, so he turned around and went back. He stopped the car, turned off the engine, and unbuckled his wristwatch. Yes, wristwatch, instead of his seatbelt.
Most everyday errors are slips. Intend to do one action, find yourself doing another. Have a person say something clearly and distinctly to you, but “hear” something quite different. The study of slips is the study of the psychology of everyday errors—what Freud called “the psychopathology of everyday life.” Some slips may indeed have hidden, darker meanings, but most are accounted for by rather simple events in our mental mechanisms.2
Slips show up most frequently in skilled behavior. We don’t make so many slips in things we are still learning. In part, slips result from a lack of attention. On the whole, people can consciously attend to only one primary thing at a time. But we often do many things at once. We walk while we talk; we drive cars while we talk, sing, listen to the radio, use a telephone, take notes, or read a map. We can do more than one thing at a time only if most of the actions are done automatically, subconsciously, with little or no need for conscious attention.
Doing several things at once is essential even in carrying out a single task. To play the piano, we must move the fingers properly over the keyboard while reading the music, manipulating the pedals, and listening to the resulting sounds. But to play the piano well, we should do these things automatically. Our conscious attention should be focused on the higher levels of the music, on style, and on phrasing. So it is with every skill. The low-level, physical movements should be controlled subconsciously.