Statistics for Maintenance Modelling: Repairable Systems

# Graphical Methods¤

Three functions of prime interest relating to repairable systems are:

- the Mean Cumulative Number of Failures (MCNF)
- the Mean Cumulative Cost Function (MCCF)
- the Rate of Occurrence of Failures (ROCOF).

Plots of the sample equivalents of these measures often reveal simple underlying power laws describing the evolution of the measures with time. The MCNF and MCCF may be plotted directly against time, or perhaps more usefully, the
ln MCNF or ln MCCF against ln t. Then an underlying power law may be estimated by drawing a straight line through the points. If more than one identical system is considered, then the number of failures is averaged over the systems in operation.