A study of the application of the lognormal distribution to corrective maintenance repair time

Abstract

The usual mathematical formulation of availability assumes an exponential distribution for failure and repair times. While such an assumption is sometimes correct for reliability, it is not valid for maintainability. This study was conducted primarily in order to verify that the lognormal distribution is a suitable descriptor for corrective maintenance repair times, and to estimate the error caused in assuming an exponential. distribution for availability and maintainability calculations when in fact the distribution is lognormal. Approximately 20 sets of existing maintainability demonstration repair time data, of essentially electronic systems, were analyzed using the methods of probability plotting and statistical testing for distributional assumption. The results show that the lognormal distribution assumption cannot be rejected in most of the cases, while the exponential distribution is rejected. However, the error caused when assuming an exponential distribution for MTTR is found to be negligible.