Failure mode criticality is calculated by the formula:

Where:

Cm=Represents the criticality number for the failure mode.

β=Represents the conditional probability of loss of function or mission, or failure effect probability.

α=Represents the failure mode ratio (for an item, Σα=1).

λp=Represents the part failure or hazard rate.

t=Represents duration of applicable mission phase, usually expressed in hours or number of operating cycles.

The λpt can be replaced with the failure probability, 1–exp(-αλpt, if α=1 and λpt<<1.

Item criticality is calculated by the formula:

Where:

Cr=Represents the criticality number for the item.

Cm=Represents the criticality number for the failure mode.

The specific type of system failure is expressed by the severity classification of the item’s failure modes. For a particular severity classification and mission phase, the Cr for an item is the sum of the failure mode criticality numbers, Cm, under the severity classification. Item criticality may also be calculated using the following formula:

Where:

Cr=Represents the criticality number for the failure mode.

n=Represents the failure modes in the items that fall under a particular criticality classification (1, 2, 3, , j).

j=Represents the last failure mode in the item under the criticality classification.

β=Represents the conditional probability of loss of function or mission, or failure effect probability.

α=Represents the failure mode ratio (for an item, Σα=1).

λp=Represents the part failure or hazard rate.

t=Represents duration of applicable mission phase, usually expressed in hours or number of operating cycles.

Worst-case or pessimistic reliability values should always be used as input assumptions for failure modes that are identified as critical, or which might be critical if the pessimistic assumptions prove to be realistic. Generally, the more critical the failure mode, the more pessimistic the worst-case reliability assumptions should be. Computerised FMEA software greatly facilitates this type of sensitivity analysis.