Criticality analysis is a quantitative assessment of the relative importance of failure modes and effects.
Criticality analysis is usally applied only to those elements of an assembly or system whose failure modes have significant effects and/or high probability of occurrence. It is a useful supplement to FMEA, provided reliable data are available. The results of a criticality analysis may be incorporated on the failure modes and effects analysis form, or they may be presented in a criticality matrix or a critical items list.
The formulas for calculating criticality are based on three distinct probability values:
These three values are used to calculate two criticality numbers:
As with other probabilistic calculations, the difficulty in performing criticality analysis stems from obtaining reliable data for the factors in the equation. They are best obtained from generic failure rate data as published in various reliability handbooks. Unless both the failure modes and these factors are determined accurately, the calculation may not be worth the effort. As time passes, the volume and accuracy of failure rate data and the values of the modifying factors will increase, which will make the analysis useful.
Another limiting factor is that the influences of other failures and human interactions are difficult to evaluate. Therefore, only a small fraction of FMEAs written use criticality analysis.