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Awareness of risk is probably greater today than it has ever been, and the assessment and management of risk-whether or not we think of it as risk-has become common in the lives of many of us. The first part of this paper notes that risk is never about predictable losses, or about either predictable or unpredictable gains; nor is risk a probability. Rather, risk is either a condition of, or a measure of, exposure to misfortune-more concretely, exposure to unpredictable losses. However, as a measure, risk is not one-dimensional-it has three distinct aspects or “facets” related to the anticipated values of unpredictable losses. The three facets are Expected Loss, Variability of Loss Values, and Uncertainty about the Accuracy of Mental Models intended to predict losses. This paper illustrates these three facets of risk by the use of three simple card-game “thought experiments”.

Redundancy in a safety-critical system has the potential for greatly improving safety. However, in physical real life that potential cannot be realized if failure of an element of one subsystem can sometimes be physically related to failure of an element of another subsystem intended to be redundant with respect to the first. Two real-life element failures can be related to each other either because (1) one failure cascaded to cause the other, or (2) an abnormal event external to both elements caused both elements to fail. System designers and safety analysts should therefore be aware of all three types of real-life failure pairs: (1) unrelated, (2) cascading/consequential related, and (3) common-external-cause related. It is the possibility of occurrence of the latter two types of real-life failure pairs which is responsible for probabilistic dependencies between failures in probabilistic safety analyses.