Although the failure rate, formula_1, is often thought of as the probability that a failure occurs in a specified interval given no failure before time formula_2, it is not actually a probability because it can exceed 1. Erroneous expression of the failure rate in % could result in incorrect perception of the measure, especially if it would be measured from repairable systems and multiple systems with non-constant failure rates or different operation times. It can be defined with the aid of the reliability function, also called the survival function, formula_3, the probability of no failure before time formula_2.