Human Reliability Assessment in Context

Human reliability assessment (HRA) is the common name for an assortment of methods and models that are used to predict the occurrence of ‘human errors’. While the origin of HRA is in Probabilistic Safety Assessment (PSA), HRA is increasingly being used on its own both as a way to assess the risks from ‘human error’ and as a way to reduce system vulnerability. According to [1] the three principal functions of HRA are “identifying what errors can occur (Human Error Identification), deciding how likely the errors are to occur (Human Error Quantification), and, if appropriate, enhancing human reliability by reducing this error likelihood (Human Error Reduction)” [1]. Practically all HRA methods and approaches share the assumption that it is meaningful to use the concept of a ‘human error’, hence also meaningful to develop ways of estimating ‘human error’ probabilities. As a consequence of this, numerous studies have been performed to produce data sets or databases that can be used as a basis for determining ‘human error’ probabilities. This view prevails despite serious doubts expressed by leading scientists and practitioners from HRA and related disciplines. A comprehensive criticism of HRA [2], for instance, pointed out that many HRA approaches are based on highly questionable assumptions about human behaviour. This view is supported by the experience from extensive studies of human performance in accidents, which conclude that: ... “human error” is not a well defined category of human performance. Attributing error to the actions of some person, team, or organization is fundamentally a social and psychological process and not an objective, technical one. [3] Although the concept of ‘human error’ itself is the subject of much debate, it is not the intention to go into that here (but see [3, 4, 5, 6]). For the purpose of this discussion a ‘human error’ will simply be defined as an identifiable human action that in retrospect is seen as being the cause of an unwanted outcome. (Needless to say, even the concept of a cause can be the subject of dispute, not least when it comes to the description of accidents [8].)