Highly reliable systems are not necessarily safe and highly safe systems are not necessarily reliable. Reliability and safety are different qualities and should not be confused. In fact, these two qualities often conflict. Increasing reliability may decrease safety and increasing safety may decrease reliability. One of the challenges of engineering is to find ways to increase safety without decreasing reliability. For example, some ways to reduce the accident rate on aircraft carriers would be to slow down the landing rates, only allow landing in the most perfect weather and the most ideal conditions, and only allow the most experienced pilots to make the landings. These operational conditions would most likely conflict with the achievement of other goals, such as training for combat.Reliability in engineering is defined as the probability that a component satisfies its specified behavioral requirements over time and under given conditions. If a human operator does not follow the specified procedures, then they are not operating reliably. In some cases that can lead to an accident. In other cases, it may prevent an accident when the specified procedures turn out to be unsafe under the particular circumstances. Examples abound of operators ignoring prescribed procedures in order to prevent an accident [Leveson, 1995]. At the same time, accidents have resulted precisely because the operators did follow the predetermined instructions provided to them in their training. When the results of deviating from procedures are positive, operators are lauded but when the results are negative, they are punished for being unreliable. HRO researchers [Weick, 1987; Roberts, 1990; Roberts, 1990; La Porte, 1991; Schulman, 1993; Weick, 1993; Weick, 1999] correctly point out the need for operators to sometimes break the rules in order to prevent an accident, but incorrectly label their
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