A note on reliability estimation of functionally diverse systems

It has been argued that functional diversity might be a plausible means of claiming independence of failures between two versions of a system. We pres ent a model of functional diversity, in the spirit of earlier models of div ersity such as those of Eckhardt and Lee, and Hughes. In terms of the model , we show that the claims for independence between functionally diverse sys tems seem rather unrealistic. Instead, it seems Likely that functionally di verse systems will exhibit positively correlated failures, and thus will be less reliable than an assumption of independence would suggest. The result does not, of course, suggest that functional diversity is not worthwhile; instead, it places upon the evaluator of such a system the onus to estimate the degree of dependence so as to evaluate the reliability of the system. (C) 1999 Elsevier Science Ltd. All rights reserved.