Category | Range (failures per year) | Catastrophic | Critical | Marginal | Negligible |
Frequent | >10−3 | I | I | I | II |
Probable | 10−3 to 10−4 | I | I | II | III |
Occasional | 10−4 to 10−5 | I | II | III | III |
Remote | 10−5 to 10−6 | II | III | III | IV |
Improbable | 10−6 to 10−7 | III | III | IV | IV |
Incredible | <10−7 | IV | IV | IV | IV |