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Criticality Assessment Methods
The numeric values that reflect the criticality of failure modes can be calculated in a variety of ways. The following table describes criticality assessment methods and indicates their sources. By sorting or grouping the results calculated by any of these methods, you can easily identify the critical modes that must be addressed to reduce failure risk. Because all of these methods provide valuable criticality values, the FMEA module supports the use of all or any combination of them. The many different calculations built into the FMEA module are described in FMEA Calculation Equations. You can also define custom RPN calculations in the Project’s Calculation file. For more information, see Calculation File.
Assessment Method
Description
Source
Mode Criticality
This method is based on actual failure rate values and is often used to assess failure mode risk for hardware components. If the failure rate of a component is established in some manner, you can assign a mode percentage to each possible failure mode of the component.
For example, assume a component has two failure modes. If one of these modes accounts for failures 75% of the time, you assign 75 to this mode and 25 to the other mode. (Databases that provide lists of failure modes for components and their associated mode percentages are available.)
You must then assign a failure effect probability for each mode, which is the likelihood of occurrence for the failure effect. Using this information, a mode failure rate can be calculated by multiplying the failure rate by the mode percentage. The equation for mode criticality is:
Mode Criticality = Failure Effect Probability * Mode Failure Rate * Operating Time of System
MIL-STD-1629
Risk Priority Numbers (RPNs)
This method is typically based on severity, occurrence, and detection values. These values are determined by the lists attached to the following FMEA fields: Severity, Detection, and Occurrence. For more information, see Severity, Occurrence, Detection, and Risk Level Lists. The built-in RPN calculation equation is:
RPN = Severity * Occurrence * Detection
Depending on the parameter, the highest score indicates the most severe, most likely to occur, or least likely to be detected failure mode. When the military scales from the MIL-STD-1629 standard are used, RPN values range from 1 (1 * 1 * 1) to 10,000 (10 * 100 * 10). When the 10-point scales common to automotive FMEA standards are used, RPN values range from 1 (1 * 1* 1) to 1,000 (10 * 10 * 10).
After RPNs are calculated and sorted, you can set specific RPN values as cutoffs for high, medium, and low risk ranges.
SAE, AIAG, and Ford automotive standards
Criticality Rank
This graphing method assesses failure mode risk in terms of severity and the probability of occurrence. Criticality rank is defined by going through all failure modes to find non-dominated failure modes. Non-dominated failure modes are those not outranked in terms of severity or probability of occurrence. The first set of non-dominated failure modes is assigned a rank of 1, and the next level of non-dominated modes is assigned a rank of 2, and so on. This procedure continues until all failure modes have been ranked.
SAE FMEA 5580
Risk Level
This method is based on plotting severity and occurrence values for failure modes to see which fall into high, medium, and low risk levels. The concept, known as a criticality or risk matrix, was introduced by Paul Palady in his book, FMEA - Failure Modes & Effect Analysis - Predicting & Preventing Problems Before They Occur. While Palady’s matrix plots failure modes by severity and occurrence and then draws lines to show boundaries for three risk levels, you can plot the values for any two or three risk assessment fields. For more information, see Generating a FMEA Risk Matrix Graph.
FMEA - Failure Modes & Effect Analysis - Predicting & Preventing Problems Before They Occur