Component relevancy in multistate reliability models

The concept of relevancy for components in multistate reliability models is the main condition for the underlying models to be coherent. The authors review existing component relevancy conditions and their structure functions, investigate their interrelationships and introduce a unified relevancy condition. They discuss the mathematical formulation of these relevancy conditions and their interrelationships. They introduce a unified criterion for defining the relevancy conditions shown to be special cases of the unified form. The effect of these relevancy conditions on the well known principle used by design engineers, that redundancy at the component level is preferable to redundancy at the system level is studied. Some counter-examples for the inverse implications between relevancies are given