Criticality Analysis

[Please note that the following article — while it has been updated from our newsletter archives — may not reflect the latest software interface and plot graphics, but the original methodology and analysis steps remain applicable.]

Criticality Analysis is another method of risk assessment that can be used in conjunction with an FMEA. MIL-STD-1629A describes the requirements for two types of failure modes, effects and criticality analysis (FMECA): quantitative and qualitative.

To perform a quantitative criticality analysis, the analysis team must: 

• Identify the functions, failures, effects and causes for each item of interest. 

• Define the reliability/unreliability for each item and use it to estimate the expected number of failures at a given operating time. 

• Identify the portion of the item’s unreliability that can be attributed to each potential failure mode. 

• Rate the probability of loss (or severity) for the effects of each failure mode, using a number from 0 to 1. 
• Calculate the criticality for each potential failure mode by multiplying the three factors: Expected Failures x Mode Ratio of Unreliability x Probability of Loss. 
• Calculate the criticality for each item by obtaining the sum of the criticalities for each failure mode that has been identified for the item. 

A qualitative criticality analysis, as described in the military standard, is similar to the Risk Priority Number (RPN) method. The analysts use predefined rating scales to rate the likelihood of occurrence for each failure mode and the severity of the potential effects of failure. However, the probability of prior detection is not considered. A matrix with severity on the horizontal and occurrence on the vertical axis can be used to compare failure modes from this analysis.