Violations arise from motivational factors and fall into four types:
routine (or corner-cutting) violations, thrill-seeking or optimizing violations, necessary violations and exceptional violations. Unsafe acts are of two distinct types: errors and violations. Errors arise from informational problems and fall into three categories:
- skill-based slips and lapses,
- rule-based mistakes, and
- knowledge-based mistakes.
Everyone makes errors.
They are an intrinsic part of the human condition, but certain situations are guaranteed to increase the chances that errors will be made. These are called (here) errorenforcing conditions. When they occur in the presence of dangers, there is a heightened risk of injury or damage.
Slips and Lapses
These are actions-not-as-planned.
Absent-minded slips arise from inattention.
Lapses are failures of memory – when we forget to carry out an intended action or forget that we have already performed a particular action.
Mistakes
Here the actions go as planned, but the plan itself is inadequate to achieve the desired goal. Mistakes occur when we are trying to solve problems or are dealing with unexpected departure from routine. There are two kinds of mistakes:
- rule-based, when we are dealing with a familiar problem but apply the wrong rule, or
- knowledge-based, in which we have run out of ready-made solutions and have to think on our feet in novel circumstances.
Conditions Promoting Errors
Slips and lapses are most likely to happen when we are performing habitual actions in familiar surroundings. Almost invariably, our minds are on something other than the task in hand. People are very good at performing routine tasks so long as things remain unchanged.
Two conditions in particular provoke slips – one psychological and the other situational:
• The psychological condition is attentional capture. This where an individual is so distracted or preoccupied with something other than the task in hand that they have little or no attentional capacity to monitor the progress of their current actions.
• The situational condition is change. This can be either a change in the nature of the task or in the circumstances in which the task is performed. The most error-enforcing changes are those in which there is only a partial alteration in the circumstances of a familiar task. In other words, most features of the task remain the same, but one or two significant aspects are changed. The worst situation is where these changes demand actions that are quite different from those normally required.
Given these two conditions, the nature of the slip is predictable – people will do what they customarily do in those circumstances, having failed to attend to the features that require them to modify their actions. Errors tend to take forms that are more familiar, more frequent in context than those that were either intended or required. If you want to know what kind of error a person will make in a particular changed circumstance, then consider what he or she did before the change was made. In summary, a very common kind of slip, perhaps the commonest, is a strong-habit intrusion.
Some of the mental factors associated with making mistakes are different from those for slips, while others are very similar. One important difference between mistakes and slips is that in the former case, we are usually aware that we have a problem or some departure from the expected. In short, we are operating in a conscious rather than an automatic mode. Rule-based mistakes, however, usually share with slips the fact that the error takes a frequent and well-used course
of action, especially if they are solutions to familiar problems and have served us well in the past. It is only natural that they spring readily to mind when we encounter a problem that has many features in common with those that we have frequently met before. These similarities between problem solutions cause us to overlook important differences. Driving, for example, is a highly routinized activity, performed very largely on automatic pilot. If we fail to notice that the road conditions have become slippery, we are likely to hit the brakes in an emergency rather than steering out of trouble.
Rule-based mistakes come in two basic forms:
- either we can misapply a normally good rule or
- we can apply a bad rule.
In learning a job, many people acquire bad habits or bad rules. Often these bad habits go unpunished and become a fixed part of a person’s skill repertoire. But sometimes the circumstances are less tolerant. Consider the case of the technician rewiring a signal box on the day before the Clapham rail disaster. He had the practice of bending wires back rather than removing them. In this instance, the bent wires uncoiled and made contact, causing the signal to fail green, allowing a commuter train (the next morning) to crash into the back of a stationary train. The debris derailed a train coming in the opposite direction.
In the case of knowledge-based mistakes we have exhausted our stock of pre-packaged solutions and have to tackle a novel situation by resorting to the highly fallible of business of thinking ‘on the hoof’. Although people can be quite good at coming up with fresh solutions when they have plenty of time to reflect, they generally perform badly under the more usual conditions of time pressure, strong emotion and the prospect of imminent disaster (e.g., Air France Flight 447). In this situation we are forced to make progress through trial-and-error. The military have always known this. This is why soldiers are drilled to carry out the necessary emergency actions when their weapons fail in the heat of battle.
There are at least three reasons why people perform badly when trying to solve novel problems:
- Human attention span (what is available to the conscious mind at any one time) is usually much smaller than the full scope of the problem. We only have a keyhole view of the total picture. This means that we have to shift our attention over the full range of the ‘problem space’. As we do this, information spills out of short-term memory and we forget what we have just done or thought about.
- When we encounter a totally new situation, we have to resort to a ‘mental model’ (a picture in the mind’s eye). This is almost always incomplete or inaccurate, or both.
- The two items above relate to the way we process information. The third reason has to do with emotions. Stress, the awareness of danger, will act to cloud thought. In particular, it causes our actions to become more stereotyped. We very easily give up the effortful business of thinking and resort to a familiar and comforting sequence of habitual actions.
Source: Organizational Accidents Revisited, James Reason, 2016