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 ergonomics (human factors) 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.