Reliability-importance and dual failure-mode components

There are several measures of the importance of a component within a system in determining system reliability; these measures include structural importance, marginal reliability-importance. Some reliability-importance, and link importance. These measures are reviewed and then related to each other. Extensions are proposed to each importance measure to cover reliability models in which the components have two failure-modes rather than the conventional one failure-mode; these extensions have properties similar to those in the one failure-mode model.