rors occur more frequently than shared errors and that failures to detect errors occur more often than failures to indicate or correct.

In this same study, the investigators identify internal and external factors that contribute to the errors made. They define these contributors as performance-shaping factors (PSFs), which include external factors such as darkness, temperature, and high work requirements that are shared by all team members working in the same environment. Internal PSFs include high stress, excessive fatigue, and deficiencies in knowledge and skill. According to Sasou and Reason (1999), internal PSFs are often influenced by external factors and may vary across individuals even under the same set of circumstances. Team PSFs are a third potential contributor to error. These include, for example, lack of communication, inappropriate task allocation, and excessive authority gradient (Sasou and Reason, 1999).

In their review of adverse events in the nuclear, aviation, and shipping industries, Sasou and Reason (1999) found the most common team PSF to be failure to communicate. Failure to communicate resulted in the inability to detect both individual and shared errors. Excessive professional courtesy, overtrusting, an air of confidence, and excessive belief were additional factors. Inadequate resources and deficient task management created errors and also led to detection failures. Excessive authority gradient was the most dominant factor in failures to indicate and correct errors, although excessive professional courtesy also led to team member reluctance to challenge error makers. Shared errors commonly occurred during the human–machine interface, where low task awareness, low situational awareness, and excessive adherence to overreliance on established practices contributed to mistakes. Failures to detect were influenced by deficiencies in communication and resource/task management, excessive authority gradient, and excessive belief. Failures to indicate/correct were influenced by excessive authority gradient, excessive professional courtesy, and deficiency in resource/task management. Based on these findings, the authors recommend that team error-reduction strategies focus on clarifying team member responsibility and accountability and on improving interpersonal skills performance. This includes efforts both to maximize communication success and to provide constructive feedback to established and well-respected team members.

A second theory of team behavior and safety processes proposes that four boundaries of safe or acceptable practice are evident in systems—physical, psychological, social, and economic (Bea, 1998). Individuals within systems function within a “safety space” created by these four boundaries and take action to withdraw when they perceive they are approaching one of the unsafe areas. In this model, the physical boundary reflects conditions in which the work or effort required is perceived to be



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