On The Specification of Repair Time Requirements

When specifying maintainability requirements, it is widely accepted practice (virtually universal in US military and DoD specifications) to specify the mean-time-to-repair and a `maximum' quantile of the repair time distribution. This practice is unsatisfactory for the lognormal distribution of repair times because: 1) As is well-known, it may be that for a given pair (mean, quantile) there is no lognormal distribution. 2) As is not so well-known, for a given pair (mean, quantile) there will be two lognormal distributions, if there are any. In short, the specification of a mean and a quantile does not uniquely determine a single lognormal distribution.