The problem of estimating equipment serviceability when the apparatus consistsof a large number of individual components (possibly with standby features) islikely to be of increased interest now that navigational thought is being concentratedquite as much on the design of instruments the navigator uses as on theproblems of finding position at sea or in the air. The resulting exercise in combiningprobabilities of component failure is a fairly straightforward one but inview of the accelerating interest in the matter (see, for example, Cluley's articlein this Journal, 15, 387) a brief systematic treatment might be of interest toworkers in these fields.