Usefulness of MTTF of s-independent case in other cases

It is a plausible conjecture that for many cases of statistical dependence, the system MTTF (mean time to failure) is nearly that for statistically independent components. The author deals with Markov k -out-of-n:F systems, for which the degree to which the above conjecture is true could be clarified satisfactorily. In order to get an approximate value of system unavailability too, it is necessary to find an approximate value of MTTR. This was easy for the above special systems, but can be difficult in general. Mainly, it can be shown that the approximate system MTTF found from the easily calculated mean failure-frequency of the statistically independent case is also useful for certain cases of statistical dependence