Outage times in fault-tolerant systems

The author gives sample field outage-time data sets from two fault-tolerant systems and discusses their statistical analysis. He uses probability plots to choose a distributional model for the data and observes that the commonly used exponential model does not fit the data. Instead a distribution is required that has a heavier tail, i.e., more variability and more long outages. A lognormal model is fitted to the data. These fits can then be used in discrete-event simulations or other analytic modeling of system availability.<>