ABSTRACT
This chapter deals with the subject of human reliability assessment (HRA). HRA may be considered a subdiscipline of ergonomics or human factors (these terms are used interchangeably in this chapter), but it emanates also from the fields of reliability engineering and risk assessment, and is therefore a hybrid discipline. HRA is fundamentally the analysis of human failures. Unlike accident analysis, however, HRA is prospective or predictive — it is concerned with determining what can go wrong, before it happens. This is no trivial task. HRA also not only tries to determine what can go wrong (i.e., human errors), but also how likely it is to go wrong, i.e., it predicts the probabilities of different errors and failures occurring. Furthermore, since HRA has become more linked to psychology and ergonomics over the last decade and a half, it has focused on how human failures occur, and what factors cause them or increase their likelihood of occurrence. Therefore, based on such analysis, it then becomes possible to determine how to prevent such errors from occurring at all, or at least to decrease their likelihood. HRA, broadly speaking, can therefore be seen to have three interlinked functions:
1. Determination of what can go wrong (human error identification) 2. Quantification of the probabilities of errors (human reliability quantification) 3. Reduction of error likelihood (error reduction analysis)
HRA is most commonly used in a risk assessment format, essentially determining how frequently accidental outcomes (e.g., fatalities) will occur in a given period of operation of a system (usually such predicted frequencies are very small, e.g., once in one hundred thousand years of operation). When utilized within risk assessment, HRA is effectively assessing the human contribution to risk. This contribution is integrated within the overall risk assessment framework, so that the human contribution to risk can be seen in conjunction with other contributions to risk: hardware and software failures, and environmental events. Therefore, when total risk is estimated for a system such as a chemical plant or an offshore platform, the relative contribution of human error (and human recovery capabilities) to risk can be judged by the owners, designers, and/or regulators of such a system. Sometimes human error will be seen as a major contributor to risk, and other times its role may be negligible, or at least tolerable. If, however, risk assessment and HRA do show that human error is of significant concern, there will be the need for more human factors effort to improve the designed operator support systems (interfaces, training, procedures, etc.). HRA can therefore lead to the determination of the adequacy, from a safety perspective, of the human factors considerations designed into a system.
