It is often desirable to be able to quantify the probability of occurrence of each event and each cut set. To perform this task, reliability and maintainability information such as failure probability, failure rate, or repair rate must be available. Information about the cut sets obtained in the qualitative stage of the analysis can then be used to compute the unavailability and unreliability of the system. For event trees, unavailability and unreliability are calculated rather than availability and reliability. This is because event trees are typically organized around failures.