Statistics for Maintenance Modelling: Continuous Lifetime Distributions 1

# Introduction¤

The next two sessions introduce the distributions that are most commonly fitted to reliability lifetime data. The session after that then discusses how to carry out the fitting. As with all model fitting, we are not saying that the lifetime data exactly matches the chosen distribution, but rather that the distribution is a very good approximation to the data. Fitting a distribution to the data assists in making good Asset Management decisions as it smoothes some of the sampling noise and fills in gaps in the data coverage. Hence there are three main criteria involved in choosing a distribution:

1. Consideration of any physical characteristics of the problem that link with a particular distribution, e.g. is there a reason to believe that the hazard rate is constant? However this kind of argument is often only appropriate for simple items.
2. How well the distribution fits the observed data.
3. How simple and easy to use the distribution is.