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.