Explicit and implicit inclusion of time in the definitions of risk and reliability

Abstract

In safety, understanding of risk and reliability is generally not possible without reference to time. For example, regarding the risk of some activity or situation, focus is on what might happen in future time in relation to the potential for negative outcomes or consequences. Currently, several definitions of risk are available, indicating a lack of consensus on how to define this, also within international standards. A common characteristic, is that neither of the definitions are explicitly capturing time. In contrast, the concept of reliability is explicit on this, focussing on failure-free performance for a given time interval, indicating a discrepancy and inconsistency between the concepts. There are different temporal aspects and key argumentation for and against inclusion of time in the definition of risk. We discuss the relevance to the understanding and interpretation, and conclude, that it is insufficient to assume time strictly implicitly as part of the possible consequences. As for the reliability concept, the element of time matters to the understanding of risk as a concept, and contributes to a more specific description of risk. A more appropriate definition of this concept that allows for this element to be explicitly captured is suggested in this article.