When an item is repairable, a reliability measure known as MTTR (mean time to repair) can be calculated. MTTR represents the downtime required to repair an item that has failed and put it back into operation. MTTR can be calculated by performing a maintainability prediction as described in Windchill Maintainability. MTTR can also be specified. The calculated or specified MTTR value is used when calculating availability to determine the probability that the item will be operable when needed.