Criticality refers to the degree of importance or significance of a failure mode based on the severity of its potential consequences, the likelihood of its occurrence and how easy it is to detect.

RPN

Criticality is expressed via the Risk Priority Number (RPN) - a numerical value that describes the potential impact and likelihood of occurrence. The RPN is a product of three factors: Severity (S), Likelihood (L), and Detectability (D).

The formula for calculating RPN is typically expressed as:
RPN = Severity × Likelihood × Detectability

Here is a general approach to calculating criticality (RPN):

1. Identify Consequences: For each failure mode, identify and categorize the consequences in terms of safety, environmental impact, operational impact, and economic impact.
2. Assign Severity Ratings: Assign severity ratings to each consequence category based on the potential impact. Use a numerical scale or pre-defined descriptive terms (e.g., low, medium, high) to quantify the severity. Take the largest of these severity ratings or use a weighted rating, if some consequences are deemed more critical than others.
3. Estimate Likelihood of Failure: Assess the likelihood or probability of the failure mode occurring. Consider historical data, expert opinions, and other relevant information.
4. Assign Likelihood Ratings: Assign likelihood ratings to each failure mode based on the estimated probability. Use a numerical scale or pre-defined descriptive terms.
5. Define how easy it is to Detect the Failure: Assess how evident the failure will be if it occurs. Consider evident consequences and any protective devices that may provide alerts.
6. Assign Detectability Ratings: Assign detectability ratings to each failure mode based on the estimated detectability. Use a numerical scale or pre-defined descriptive terms.
7. Calculate Criticality (RPN): Calculate the RPN score for each failure mode by multiplying the assigned severity rating by the assigned likelihood rating and detectability rating.

SOD

Criticality is also often represented by the SOD - a combination of the Severity (S), the probability of Occurrence (O) and the Detectability (D). However, instead of multiplying the numbers, they are each displayed. For example: S2-O4-D4, or just 244.