ABSTRACT
The term “human reliability” is usually defined as the probability that a person will correctly perform some system-required activity during a given time period (if time is a limiting factor) without performing any extraneous activity that can degrade the system. The historical background for the development of the set of methods that are commonly referred to as Human Reliability Analysis (HRA) was the need to describe incorrect human actions in the context of Probabilistic Risk Assessment (PRA) or Probabilistic Safety Analysis (PSA). The premises for HRA were, and are, therefore that it must function within the constraints defined by PRA/PSA and specifically that it can produce the human action probabilities that are needed by the PRA/PSA. The accident sequence that is analyzed by a PRA/PSA is typically represented as an event tree (see Figure 1). A node in the sequence of events that may lead to the accident represents a specific function, task, or activity that can have two different outcomes, usually denoted success and failure. A node can either represent the function of a technical system or component, or the interaction between an operator and the process. For example, if the analysis considers the sequence of events that are part of landing an aircraft, the event “timely extraction of flaps,” which is an action that must be taken by the pilot, is represented by a node. From the perspective of the PRA/PSA there is a need to know whether it is likely that an event will succeed or fail, and further to determine the
probability of failure in order to calculate the combined probability that a specific outcome or end state will occur. If the node represents the function of a mechanical or electronic component, the failure probability can, in principle, be calculated based on engineering knowledge alone. If the node represents the interaction between an operator and the process, engineering knowledge must be supplemented by a way of calculating the probability that the human, as a “component,” will fail. Historically, the role of HRA has been to provide the foundation for calculating this probability. The sought for value has traditionally been called a human error probability (HEP), but as the following will show this is both a misunderstood and misleading term.
