The Design of Everyday Things (32 page)

FIGURE 5.2.
   
Where Slips and Mistakes Originate in the Action Cycle.
Figure A
shows that action slips come from the bottom four stages of the action cycle and mistakes from the top three stages. Memory lapses impact the transitions between stages (shown by the X's in
Figure B
). Memory lapses at the higher levels lead to mistakes, and lapses at the lower levels lead to slips.

Slips are the result of subconscious actions getting 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 mistakes. 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. This leads to mistakes that can be difficult to discover, let alone eliminate.

The Classification of 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, his wristwatch, instead of his seatbelt
.

The story illustrates both a memory-lapse slip and an action slip. The forgetting of the briefcase is a memory-lapse slip. The unbuckling of the wristwatch is an action slip, in this case a combination of description-similarity and capture error (described later in this chapter).

Most everyday errors are slips. Intending to do one action, you find yourself doing another. When a person says something clearly and distinctly to you, you “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.” Freud believed that slips have hidden, dark meanings, but most are accounted for by rather simple mental mechanisms.

An interesting property of slips is that, paradoxically, they tend to occur more frequently to skilled people than to novices. Why? Because slips often result from a lack of attention to the task. Skilled people—experts—tend to perform tasks automatically, under subconscious control. Novices have to pay considerable conscious attention, resulting in a relatively low occurrence of slips.

Some slips result from the similarities of actions. Or an event in the world may automatically trigger an action. Sometimes our thoughts and actions may remind us of unintended actions, which we then perform. There are numerous different kinds of action slips, categorized by the underlying mechanisms that give rise to them. The three most relevant to design are:

       
•
  
capture slips

       
•
  
description-similarity slips

       
•
  
mode errors

CAPTURE SLIPS

          
I was using a copying machine, and I was counting the pages. I found myself counting, “1, 2, 3, 4, 5, 6, 7, 8, 9, 10, Jack, Queen, King.” I had been playing cards recently
.

The capture slip is defined as the situation where, instead of the desired activity, a more frequently or recently performed one gets done instead: it captures the activity.
Capture errors require that part of the action sequences involved in the two activities be identical, with one sequence being far more familiar than the other. After doing the identical part, the more frequent or more recent activity continues, and the intended one does not get done. Seldom, if ever, does the unfamiliar sequence capture the familiar one. All that is needed is a lapse of attention to the desired action at the critical junction when the identical portions of the sequences diverge into the two different activities. Capture errors are, therefore, partial memory-lapse errors. Interestingly, capture errors are more prevalent in experienced skilled people than in beginners, in part because the experienced person has automated the required actions and may not be paying conscious attention when the intended action deviates from the more frequent one.

Designers need to avoid procedures that have identical opening steps but then diverge. The more experienced the workers, the more likely they are to fall prey to capture. Whenever possible, sequences should be designed to differ from the very start.

DESCRIPTION-SIMILARITY SLIPS

          
A former student reported that one day he came home from jogging, took off his sweaty shirt, and rolled it up in a ball, intending to throw it in the laundry basket. Instead he threw it in the toilet. (It wasn't poor aim: the laundry basket and toilet were in different rooms.)

In the slip known as a description-similarity slip, the error is to act upon an item similar to the target. This happens when the description of the target is sufficiently vague. Much as we saw in
Chapter 3
,
Figure 3.1
, where people had difficulty distinguishing among different images of money because their internal descriptions did not have sufficient discriminating information, the same thing can happen to us, especially when we are tired, stressed, or overloaded. In the example that opened this section, both the laundry basket and the toilet bowl are containers, and if the description of the target was sufficiently ambiguous, such as “a large enough container,” the slip could be triggered.

Remember the discussion in
Chapter 3
that most objects don't need precise descriptions, simply enough precision to distinguish the desired target from alternatives. This means that a description that usually suffices may fail when the situation changes so that multiple similar items now match the description. Description-similarity errors result in performing the correct action on the wrong object. Obviously, the more the wrong and right objects have in common, the more likely the errors are to occur. Similarly, the more objects present at the same time, the more likely the error.

Designers need to ensure that controls and displays for different purposes are significantly different from one another. A lineup of identical-looking switches or displays is very apt to lead to description-similarity error. In the design of airplane cockpits, many controls are shape coded so that they both look and feel different from one another: the throttle levers are different from the flap levers (which might look and feel like a wing flap), which are different from the landing gear control (which might look and feel like a wheel).

MEMORY-LAPSE SLIPS

Errors caused by memory failures are common. Consider these examples:

       
•
  
Making copies of a document, walking off with the copy, but leaving the original inside the machine.

       
•
  
Forgetting a child. This error has numerous examples, such as leaving a child behind at a rest stop during a car trip, or in the dressing room of a department store, or a new mother forgetting her one-month-old and having to go to the police for help in finding the baby.

       
•
  
Losing a pen because it was taken out to write something, then put down while doing some other task. The pen is forgotten in the activities of putting away a checkbook, picking up goods, talking to a salesperson or friends, and so on. Or the reverse: borrowing a pen, using it, and then putting it away in your pocket or purse, even though it is someone else's (this is also a capture error).

       
•
  
Using a bank or credit card to withdraw money from an automatic teller machine, then walking off without the card, is such a frequent error that many machines now have a forcing function: the card must be removed before the money will be delivered. Of course, it is then possible to walk off without the money, but this is less likely than forgetting the card because money is the goal of using the machine.

Memory lapses are common causes of error. They can lead to several kinds of errors: failing to do all of the steps of a procedure; repeating steps; forgetting the outcome of an action; or forgetting the goal or plan, thereby causing the action to be stopped.

The immediate cause of most memory-lapse failures is interruptions, events that intervene between the time an action is decided upon and the time it is completed. Quite often the interference comes from the machines we are using: the many steps required between the start and finish of the operations can overload the capacity of short-term or working memory.

There are several ways to combat memory-lapse errors. One is to minimize the number of steps; another, to provide vivid reminders of steps that need to be completed. A superior method is to use the
forcing function of
Chapter 4
. For example, automated teller machines often require removal of the bank card before delivering the requested money: this prevents forgetting the bank card, capitalizing on the fact that people seldom forget the goal of the activity, in this case the money. With pens, the solution is simply to prevent their removal, perhaps by chaining public pens to the counter. Not all memory-lapse errors lend themselves to simple solutions. In many cases the interruptions come from outside the system, where the designer has no control.

MODE-ERROR SLIPS

A mode error occurs when a device has different states in which the same controls have different meanings: we call these states
modes
. Mode errors are inevitable in anything that has more possible actions than it has controls or displays; that is, the controls mean different things in the different modes. This is unavoidable as we add more and more functions to our devices.

Ever turn off the wrong device in your home entertainment system? This happens when one control is used for multiple purposes. In the home, this is simply frustrating. In industry, the confusion that results when operators believe the system to be in one mode, when in reality it is in another, has resulted in serious accidents and loss of life.

It is tempting to save money and space by having a single control serve multiple purposes. Suppose there are ten different functions on a device. Instead of using ten separate knobs or switches—which would take considerable space, add extra cost, and appear intimidatingly complex, why not use just two controls, one to select the function, the other to set the function to the desired condition? Although the resulting design appears quite simple and easy to use, this apparent simplicity masks the underlying complexity of use. The operator must always be completely aware of the mode, of what function is active. Alas, the prevalence of mode errors shows this assumption to be false. Yes, if I select a mode and then immediately adjust the parameters, I am not apt to be confused about the state. But what if I select the mode and then get interrupted
by other events? Or if the mode is maintained for considerable periods? Or, as in the case of the
Airbus accident discussed below, the two modes being selected are very similar in control and function, but have different operating characteristics, which means that the resulting mode error is difficult to discover? Sometimes the use of modes is justifiable, such as the need to put many controls and displays in a small, restricted space, but whatever the reason, modes are a common cause of confusion and error.

Alarm clocks often use the same controls and display for setting the time of day and the time the alarm should go off, and many of us have thereby set one when we meant the other. Similarly, when time is displayed on a twelve-hour scale, it is easy to set the alarm to go off at seven
A
.
M
. only later to discover that the alarm had been set for seven
P
.
M
. The use of “
A
.
M
.” and “
P
.
M
.” to distinguish times before and after noon is a common source of confusion and error, hence the common use of 24-hour time specification throughout most of the world (the major exceptions being North America, Australia, India, and the Philippines). Watches with multiple functions have similar problems, in this case required because of the small amount of space available for controls and displays. Modes exist in most computer programs, in our cell phones, and in the automatic controls of commercial aircraft. A number of serious accidents in commercial aviation can be attributed to mode errors, especially in aircraft that use automatic systems (which have a large number of complex modes). As automobiles become more complex, with the dashboard controls for driving, heating and air-conditioning, entertainment, and navigation, modes are increasingly common.

An accident with an Airbus airplane illustrates the problem. The flight control equipment (often referred to as the automatic pilot) had two modes, one for controlling vertical speed, the other for controlling the flight path's angle of descent. In one case, when the pilots were attempting to land, the pilots thought that they were controlling the angle of descent, whereas they had accidentally
selected the mode that controlled speed of descent. The number (–3.3) that was entered into the system to represent an appropriate angle (–3.3º) was too steep a rate of descent when interpreted as vertical speed (–3,300 feet/minute: –3.3º would only be –800 feet/minute). This mode confusion contributed to the resulting fatal accident. After a detailed study of the accident, Airbus changed the display on the instrument so that vertical speed would always be displayed with a four-digit number and angle with two digits, thus reducing the chance of confusion.