In reliability, one is concerned with designing an item to last as long as possible without failure; in maintainability, the emphasis is on designing an item so that a failure can be corrected as quickly as possible. The combination of high reliability and high maintainability results in high system availability. Maintainability, then, is a measure of the ease and rapidity with which a system or equipment can be restored to operational status following a failure. It is a function of the equipment design and installation, personnel availability in the required skill levels, adequacy of maintenance procedures and test equipment, and the physical environment under which maintenance is performed. As with reliability, maintainability parameters are also probabilistic and are analyzed by the use of continuous and discrete random variables, probabilistic parameters, and statistical distributions. An example of a discrete maintainability parameter is the number of maintenance actions completed in some time t, whereas an example of a continuous maintainability parameter is the time to complete a maintenance action.
This is probably the most important distribution in reliability work and is used almost exclusively for reliability prediction of electronic equipment. It describes the situation wherein the hazard rate is constant which can be shown to be generated by a Poisson process. This distribution is valuable if properly used. It has the advantages of:
- single, easily estimated parameter (λ)
- mathematically very tractable
- fairly wide applicability
- is additive that is, the sum of a number of independent exponentially distributed variables is exponentially distributed.
Some particular applications of this model include:
- items whose failure rate does not change significantly with age.
- complex and repairable equipment without excessive amounts of redundancy.
- equipment for which the early failures or “infant mortalities” have been eliminated by “burning in” the equipment for some reasonable time period.
The failure density function is
for t > 0, where λ is the hazard (failure) rate, and the reliability function is
the mean life (θ) = 1/λ, and, for repairable equipment the MTBF = θ = 1/λ .