5.6.2 __Statistical Distributions
Used in Maintainability Models__

A smaller number of statistical distributions is used for
maintainability analysis than for reliability analysis. This may be due to the
fact that maintainability has traditionally lagged reliability theory in
development.

The most commonly used distributions for maintainability
analysis have been the normal, lognormal, and exponential. Just as the
exponential distribution has been the one most widely used in reliability
analysis of equipment/systems, the lognormal distribution is the most commonly
used for equipment/system maintainability analysis. A number of studies have
validated the lognormal as being the most appropriate for maintainability
analysis (Ref.
[25]).

However, use of other distributions
such as the Weibull and gamma is also possible, depending upon the analysis of the data and the use of “goodness
of fit” tests. Since the form and expressions for the more commonly used
distributions were previously given in Section 5.2.2, this section will
concentrate on the use of the normal, exponential, and lognormal distribution,
and give examples of their use in maintainability
analysis.