Measures of Dependability

This paper defines dependability as the probability that a system will be able to operate when needed. There are many possible measures of dependability. Three of the most important are pointwise availability---the probability that the system will be operable at a specified instant of time reliability---the probability that the system will not fail during a given interval of time, and interval availability---the expected fraction of a given interval of time the system will be operable. The probability that the system is operable at the start of the interval must be determined to evaluate reliability or interval availability. If the system's failure and repair rates do not change, this probability is easily determined. However, most systems are not required to perform their duty constantly, therefore, the system has a different set of failure and repair rates depending upon whether the system is performing its duty, is off, or is warming up. An equation is given to determine the probability that the system is operable at the start of an interval if the system repeats the series of different intervals duty, off, warming up, etc. in identical form.