The Design of Everyday Things (31 page)

It wasn't difficult for me to suggest simple changes to procedures that would have prevented most of the incidents at the utility company. It had never occurred to the committee to think of this. The problem is that to have followed my recommendations would have meant changing the culture from an attitude among the field workers that “We are supermen: we can solve any problem, repair the most complex outage. We do not make errors.” It is not possible to eliminate human error if it is thought of as a personal failure rather than as a sign of poor design of procedures or equipment. My report to the company executives was received politely. I was even thanked. Several years later I contacted a friend at the company and asked what changes they had made. “No changes,” he said. “And we are still injuring people.”

One big problem is that the natural tendency to blame someone for an error is even shared by those who made the error, who often agree that it was their fault. People do tend to blame themselves when they do something that, after the fact, seems inexcusable. “I knew better,” is a common comment by those who have erred. But when someone says, “It was my fault, I knew better,” this is not a valid analysis of the problem. That doesn't help prevent its recurrence. When many people all have the same problem, shouldn't another cause be found? If the system lets you make the error, it is badly designed. And if the system induces you to make the error, then it is really badly designed. When I turn on the wrong stove burner, it is not due to my lack of knowledge: it is due to poor mapping between controls and burners. Teaching me the relationship will not stop the error from recurring: redesigning the stove will.

We can't fix problems unless people admit they exist. When we blame people, it is then difficult to convince organizations to restructure the design to eliminate these problems. After all, if a person is at fault, replace the person. But seldom is this the case: usually the system, the procedures, and social pressures have led
to the problems, and the problems won't be fixed without addressing all of these factors.

Why do people err? Because the designs focus upon the requirements of the system and the machines, and not upon the requirements of people. Most machines require precise commands and guidance, forcing people to enter numerical information perfectly. But people aren't very good at great precision. We frequently make errors when asked to type or write sequences of numbers or letters. This is well known: so why are machines still being designed that require such great precision, where pressing the wrong key can lead to horrendous results?

People are creative, constructive, exploratory beings. We are particularly good at novelty, at creating new ways of doing things, and at seeing new opportunities. Dull, repetitive, precise requirements fight against these traits. We are alert to changes in the environment, noticing new things, and then thinking about them and their implications. These are virtues, but they get turned into negative features when we are forced to serve machines. Then we are punished for lapses in attention, for deviating from the tightly prescribed routines.

A major cause of error is time stress. Time is often critical, especially in such places as manufacturing or chemical processing plants and hospitals. But even everyday tasks can have time pressures. Add environmental factors, such as poor weather or heavy traffic, and the time stresses increase. In commercial establishments, there is strong pressure not to slow the processes, because doing so would inconvenience many, lead to significant loss of money, and, in a hospital, possibly decrease the quality of patient care. There is a lot of pressure to push ahead with the work even when an outside observer would say it was dangerous to do so. In many industries, if the operators actually obeyed all the procedures, the work would never get done. So we push the boundaries: we stay up far longer than is natural. We try to do too many tasks at the same time. We drive faster than is safe. Most of the time we manage okay. We might even be rewarded and praised for our heroic
efforts. But when things go wrong and we fail, then this same behavior is blamed and punished.

Deliberate Violations

Errors are not the only type of human failures. Sometimes people knowingly take risks. When the outcome is positive, they are often rewarded. When the result is negative, they might be punished. But how do we classify these deliberate violations of known, proper behavior? In the error literature, they tend to be ignored. In the accident literature, they are an important component.

Deliberate deviations play an important role in many accidents. They are defined as cases where people intentionally violate procedures and regulations. Why do they happen? Well, almost every one of us has probably deliberately violated laws, rules, or even our own best judgment at times. Ever go faster than the speed limit? Drive too fast in the snow or rain? Agree to do some hazardous act, even while privately thinking it foolhardy to do so?

In many industries, the rules are written more with a goal toward legal compliance than with an understanding of the work requirements. As a result, if workers followed the rules, they couldn't get their jobs done. Do you sometimes prop open locked doors? Drive with too little sleep? Work with co-workers even though you are ill (and might therefore be infectious)?

Routine violations occur when noncompliance is so frequent that it is ignored. Situational violations occur when there are special circumstances (example: going through a red light “because no other cars were visible and I was late”). In some cases, the only way to complete a job might be to violate a rule or procedure.

A major cause of violations is inappropriate rules or procedures that not only invite violation but encourage it. Without the violations, the work could not be done. Worse, when employees feel it necessary to violate the rules in order to get the job done and, as a result, succeed, they will probably be congratulated and rewarded. This, of course, unwittingly rewards noncompliance. Cultures that encourage and commend violations set poor role models.

Although violations are a form of error, these are organizational and societal errors, important but outside the scope of the design of everyday things. The human error examined here is unintentional: deliberate violations, by definition, are intentional deviations that are known to be risky, with the potential of doing harm.

Two Types of Errors: Slips and Mistakes

Many years ago, the British psychologist James Reason and I developed a general classification of human error. We divided human error into two major categories: slips and mistakes (
Figure 5.1
). This classification has proved to be of value for both theory and practice. It is widely used in the study of error in such diverse areas as industrial and aviation accidents, and medical errors. The discussion gets a little technical, so I have kept technicalities to a minimum. This topic is of extreme importance to design, so stick with it.

DEFINITIONS: ERRORS, SLIPS, AND MISTAKES

FIGURE 5.1.
   
Classification of Errors.
Errors have two major forms. Slips occur when the goal is correct, but the required actions are not done properly: the execution is flawed. Mistakes occur when the goal or plan is wrong. Slips and mistakes can be further divided based upon their underlying causes. Memory lapses can lead to either slips or mistakes, depending upon whether the memory failure was at the highest level of cognition (mistakes) or at lower (subconscious) levels (slips). Although deliberate violations of procedures are clearly inappropriate behaviors that often lead to accidents, these are not considered as errors (see discussion in text).

Human error is defined as any deviance from “appropriate” behavior. The word
appropriate
is in quotes because in many circumstances, the appropriate behavior is not known or is only determined
after the fact. But still, error is defined as deviance from the generally accepted correct or appropriate behavior.

Error
is the general term for all wrong actions. There are two major classes of error:
slips
and
mistakes
, as shown in
Figure 5.1
; slips are further divided into two major classes and mistakes into three. These categories of errors all have different implications for design. I now turn to a more detailed look at these classes of errors and their design implications.

SLIPS

A slip occurs when a person intends to do one action and ends up doing something else. With a slip, the action performed is not the same as the action that was intended.

There are two major classes of slips:
action-based
and
memory-lapse
. In action-based slips, the wrong action is performed. In lapses, memory fails, so the intended action is not done or its results not evaluated. Action-based slips and memory lapses can be further classified according to their causes.

       
Example of an action-based slip.
I poured some milk into my coffee and then put the coffee cup into the refrigerator. This is the correct action applied to the wrong object.

       
Example of a memory-lapse slip.
I forget to turn off the gas burner on my stove after cooking dinner.

MISTAKES

A mistake occurs when the wrong goal is established or the wrong plan is formed. From that point on, even if the actions are executed properly they are part of the error, because the actions themselves are inappropriate—they are part of the wrong plan. With a mistake, the action that is performed matches the plan: it is the plan that is wrong.

Mistakes have three major classes:
rule-based, knowledge-based
, and
memory-lapse
. In a rule-based mistake, the person has appropriately diagnosed the situation, but then decided upon an erroneous course of action: the wrong rule is being followed. In a knowledge-based mistake, the problem is misdiagnosed because
of erroneous or incomplete knowledge. Memory-lapse mistakes take place when there is forgetting at the stages of goals, plans, or evaluation. Two of the mistakes leading to the
“Gimli Glider” Boeing 767 emergency landing were:

       
Example of knowledge-based mistake.
Weight of fuel was computed in pounds instead of kilograms.

       
Example of memory-lapse mistake.
A mechanic failed to complete troubleshooting because of distraction.

ERROR AND THE SEVEN STAGES OF ACTION

Errors can be understood through reference to the seven stages of the action cycle of
Chapter 2
(
Figure 5.2
). Mistakes are errors in setting the goal or plan, and in comparing results with expectations—the higher levels of cognition. Slips happen in the execution of a plan, or in the perception or interpretation of the outcome—the lower stages. Memory lapses can happen at any of the eight transitions between stages, shown by the X's in
Figure 5.2B
. A memory lapse at one of these transitions stops the action cycle from proceeding, and so the desired action is not completed.