8. Assessing Product Reliability
8.1. Introduction
8.1.7. What are some basic repair rate models used for repairable systems?

## Exponential law

The Exponential Law is another flexible NHPP model An NHPP with ROCOF or intensity function given by $$m(t) = \mbox{exp} \left(\alpha + \beta t \right) \,\, ,$$ is said to follow an Exponential Law. This is also called the log-linear model or the Cox-Lewis model.

A system whose repair rate follows this flexible model is improving if $$\beta$$ < 0 and deteriorating if $$\beta$$ > 0. When $$\beta$$ = 0, the Exponential Law reduces to the HPP constant repair rate model