Regarding the thread in RISKS-24.71 and 72, the results of Knight and Leveson's famous N-version experiment show that, if any three of the replicates from among those they had written were combined in a two-out-of-three voting configuration, the resulting fault-tolerant system would have a probability of failure 19 times smaller than one of the replicates on its own. This is not as much as fully independent failure would yield, but it is a significant improvement. Peter Mellor +44 (0)20 8459 7669 Mobile: 07914 045072 MellorPeter@aol.com ------------------------------