Failure Mode Assumptions and Assumption Coverage

D. Powell



A method is proposed for the formal analysis of failure mode assumptions and for the evaluation of the dependability of systems whose design correctness is conditioned on the validity of such assumptions. Formal definitions are given for the types of errors that can affect items of service delivered by a system or component. Failure mode assumptions are then formalized as assertions on the types of errors that a component may induce in its enclosing system. The concept of assumption coverage is introduced to relate the notion of partially-ordered assumption assertions to the quantification of system dependability. Assumption coverage is shown to be extremely important in systems requiring very high dependability. It is also shown that the need to increase system redundancy to accommodate more severe modes of component failure can sometimes result in a decrease in dependability.