Human error

A human error (hereinafter, the Error) is any unintentional act of a human being working on a system that can potentially degrade this system. In other words, the Error can be defined as an individual's deviation from acceptable or desirable practice which culminates in undesirable or unexpected results.

The Error is one of the many contributing causes of risk events and a significant cause of disasters and accidents in industries such as nuclear power, aviation, space exploration, and medicine. Prevention of the Errors and/or their impact is a major contributor to reliability and safety of complex systems. Studies of human factors and ergonomics that allow for reduction of the Errors are the focus of several disciplines such as crew resource management and maintenance resource management.

Categories

Similarly to human performance, the Errors can be categorized in many ways.

Action Errors

While making the Error, a human being is performing either:

1. An incorrect task. Commonly, this Error is categorized as a mistake; OR
2. A correct task incorrectly. Commonly, this Error is categorized as a slip.

Failure-timing Errors

J.T. Reason developed the classification of unsafe acts that distinguishes between two types of errors:

1. Active failures, whose effects are felt immediately in a system. Active failures are usually the result of actions taken (or not taken) by front-line operators such as pilots, air traffic controllers, or anyone else with direct access to the dynamics of a system.
2. Latent failures, whose effects may lie dormant until triggered later, usually by other mitigating factors. Latent failures, on the other hand, are caused by those separated by time and space from the consequences of their actions in the dynamics of the system. Personnel working in vocations such as architectural design, hardware design and equipment maintenance are more prone to cause latent failures than active failures. On another hand, consider the case of a mechanic who assembled a component incorrectly which eventually led to a plane crash days or even weeks later. The defenses that should have normally caught this mistake were not in place. These defenses include proper training (the mechanic was taught to fix this particular component very informally and on-the-job), good situational awareness (the mechanic was tired from a double shift the night before), and independent inspection (the job was "pencil-whipped" to save time).

The presence of defenses or safeguards in a system can usually prevent the effects of latent failures from being felt by closing the window of opportunity during which an active failure may be committed.

Both active and latent failures may interact to create a window for accidents to occur. Latent failures set the stage for the accident while active failures tend to be the catalyst for the accident to finally occur. A good way to think of this model of accident creation is as slices of Swiss cheese. Each slice can be thought of as a defense to an accident (training, good management, teamwork, etc.) and each hole is a failure in that defense. The last slice is the final action which could serve as a defense before the accident event. The failure in that defense would constitute the active failure precipitating the accident. If the defenses to a situation contain a sufficient number of failures, which allow the holes to "line up," an accident will occur.

Differences between active and latent failures cannot be over emphasized;each type of error helps to shape the type of training required to correct them. For example, because of the immediate demands and consequences of their actions, flight personnel require training that includes the psychomotor aspects of physical skills such as improving reaction time in emergency training. The strict physical requirements for employment as a flight officer demonstrate this emphasis clearly. On the other hand, maintenance personnel may require ergonomics (human factors) and operations training to account for their susceptibility to latent failures.

In addition, the range of physical activities of maintenance personnel on the job also requires emphasis on workplace ergonomics. For example, maintenance personnel may be asked to lift heavy objects, work in awkward positions, or perform tasks in extreme weather conditions. These difficult work conditions all require knowledge of ergonomics to ensure safe, error-free performance. Though CRM and MRM share the basic concepts of error prevention, the content of what is taught is specific to what is actually performed on the job.

Skill Acquisition

A skilled task is always much more difficult to achieve when one first starts to learn it, but somehow with repeated practice the skill becomes very easy to do. This applies to complex skills even more than simple skills. An example is driving a car; at first it seems impossible to master all the concurrent tasks required and the workload appears impossible to cope with. However, after several years those same tasks generate such a low workload that the driver switches on the radio to relieve boredom.

The common models of skill acquisition are continuums that describe the skill learning process. Fitts & Posner three stage model (1967) describes initial skill learning as the cognitive stage, that gives way to the associative stage, and finally the autonomous stage.

Clearly the more complex the skill is, the longer the process takes, but it is surprising how the most seemingly impossible tasks can eventually become automatic for someone who practices enough (such as riding a unicycle while juggling clubs).

Once a skill is autonomous then it requires little or no attention to carry out, and so the process of making repeated tasks into automatic routines is one of the human brain’s most important strategies of reducing workload on known tasks, in order to release attention for use on other tasks. This process does not just apply to motor skills, but many sets of procedures that are repeated reliably, arguably including decisions. A new and novel problem requiring a decision will take a lot of effort and conscious thought, regardless how much of an expert someone is.

A final point on skill learning is that it is generally understood that skills cannot be improved without practice of some kind. At the very least, repetition is required to form the autonomous routines, improve them and maintain them.

Error Types

Errors are categorised within the umbrella term ‘unsafe acts’, which includes all errors but also other issues. An unsafe act is an action (or inaction) that leads to a safety issue. An unsafe act can happen by accident (error) it can also happen intentionally (violation).

The most widely used method to categorize unsafe acts is the is Rasmussen’s SRK taxonomy (Rasmussen 1986) and an extension of it; Reason’s generic error modelling system (GEMS).

Unsafe acts are divided into three major types: errors (Skill-based), mistakes (knowledge-based) and violations.

1. Skill-based Error; In the occurrence of an unsafe act (including an omission) as part of a learned skill or unconscious procedure then it is called a skill-based error. If this error occurred because the skilled action was inappropriate for the situation, this is called a slip (for example raising the flap lever while intending to have raised the gear lever). If on the other hand the error occurs because a skill or task step was omitted, or something was forgotten, this is a lapse (forgetting to raise the undercarriage).

One also hears the terms ‘error of commission’ and ‘error of omission’, which are effectively the same as slip and lapse. For example taking the wrong turn on a car journey because that is the turning they usually take on most other journeys (this would be called a slip, because it is an error of ‘doing’ something).

An example of a lapse would be that the driver forgot to post a letter on the way to work, because usually her journey passes the post box without stopping. This is called a lapse because it is an error of not doing something. One can see that the situational drivers to slips and lapses are similar even though the labels are different. Skill-based errors are related to attention because with little or no attention the error can occur.