FMEA Scales

Abstract

Carl and Fred discuss whether FMEA teams should use the 1–10 Severity, Occurrence, and Detection (SOD) scales in some FMEA Standards, or whether they can tailor the scales to better fit their applications.

Key Points

Join Carl and Fred as they discuss the essence of FMEA scales and how to apply them to prioritize risk.

Topics include:

Some industries use SO (Severity and Occurrence), while others use SOD (Severity, Occurrence, and Detection).
The purpose of SOD scales is to prioritize risk, not to produce objective or universally comparable numbers.
Even when teams are given a 1–10 scale, they sometimes naturally collapse it into a smaller number of categories
Published standards often provide example scales, but many organizations mistakenly treat them as mandatory.
Historically, both military and automotive FMEA standards have allowed tailoring, if approved by customer.
A problem with detailed numerical scales is false precision. Teams often cannot know whether an occurrence probability is truly one in 1,000 versus one in 10,000, yet the scale implies that level of certainty.
Recommend using the least number of gradations necessary to differentiate risks effectively.
Avoid spending excessive time debating whether a risk is a “2” or a “3” when there are much higher-priority risks demanding attention.
FMEA loses value when it becomes a numbers game or merely an exercise in filling out forms rather than making sound engineering decisions.
FMEA should remain lean, focused, and practical, concentrating on the risks that matter most.
Recommend companies publish internal FMEA procedures and scales tailored to their products and industry, rather than letting each team invent its own approach.
Severity rankings differ from Occurrence and Detection rankings because severity often relates to important consequences such as safety, loss of life, or catastrophic equipment loss, which may warrant special treatment.
Concern about use of automotive 1–10 severity scale because it can place relatively minor injuries and catastrophic loss of life into the same highest severity categories, masking important distinctions.
Standards should be viewed as guidance that can often be tailored—with company and customer agreement—to better fit the risks and decision-making needs of the organization.
The goal is not numerical precision; it is effective prioritization and better engineering decisions.

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