Design of Everyday Things [61]
Some things seem designed to cause slips. Long rows of identical switches are perfect setups for description errors. Intend to flip one switch, instead flip a similar-looking one. It happens in industrial plants, aircraft, homes, anywhere. When different actions have similar descriptions, there is a good chance of mishap, especially when the operator is experienced and well practiced and therefore not paying full attention, and if there are more important things to do.
DATA-DRIVEN ERRORS
“I was assigning a visitor a room to use. I decided to call the department secretary to tell her the room number. I used the telephone in the alcove outside the room, with the room number in sight. Instead of dialing the secretary’s phone number—which I use frequently and know very well—I dialed the room number.”
Much human behavior is automatic, for example, brushing away an insect. Automatic actions are data driven—triggered by the arrival of the sensory data. But sometimes data-driven activities can intrude into an ongoing action sequence, causing behavior that was not intended.
ASSOCIATIVE ACTIVATION ERRORS
“My office phone rang. I picked up the receiver and bellowed ‘Come in’ at it.”5
If external data can sometimes trigger actions, so, too, can internal thoughts and associations. The ringing of the telephone and knocking on the door both signal the need to greet someone. Other errors occur from associations among thoughts and ideas. Associative activation errors are the slips studied by Freud; you think something that ought not to be said and then, to your embarrassment, you say it.
LOSS-OF-ACTIVATION ERRORS
“I have to go to the bedroom before I start working in the dining room. I start going there and realize as I am walking that I have no idea why I am going there. Knowing myself, I keep going, hoping that something in the bedroom will remind me.... I get there but still cannot recall what I wanted... so I go back to the dining room. There I realize that my glasses are dirty. With great relief I go back to the bedroom, get my handkerchief, and wipe my glasses clean.”
One of the more common slips is simply forgetting to do something. More interesting is forgetting part of the act, remembering the rest, as in the story above where the goal was forgotten, but the rest of the action continued unimpaired. One of my informants walked all the way through the house to the kitchen and opened the refrigerator door; then he wondered why he was there. Lack-of-activation errors occur because the presumed mechanism—the “activation” of the goals—has decayed. The less technical but more common term would be “forgetting.”
MODE ERRORS
“I had just completed a long run from my university to my home in what I was convinced would be record time. It was dark when I got home, so I could not read the time on my stopwatch. As I walked up and down the street in front of my home, cooling off, I got more and more anxious to see how fast I had run. I then remembered that my watch had a built-in light, operated by the upper right-hand button. Elated, I depressed the button to illuminate the reading, only to read a time of zero seconds. I had forgotten that in stopwatch mode, the same button [that in the normal, time-reading mode would have turned on a light] cleared the time and reset the stopwatch.”
Mode errors occur when devices have different modes of operation, and the action appropriate for one mode has different meanings in other modes. Mode errors are inevitable any time equipment is designed to have more possible actions than it has controls or displays, so the controls must do double duty. Mode errors are especially 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