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FMEA Severity, Occurrence and Detection Rating Scales

Severity, occurrence and detection ratings are only useful when everyone on the team uses the same scale. This reference shows the familiar 1 to 10 criteria side by side so you can rate consistently, train new engineers and paste agreed scales into your FMEA procedure.

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Rating	Severity level	Severity criteria (summary)	Occurrence level	Example failure rate	Detection level	Detection criteria (summary)
10	Hazardous, no warning	Safety or regulatory effect with no warning	Very high	1 in 10 or more	Almost impossible	No known control can detect the failure
9	Hazardous, with warning	Safety or regulatory effect with warning	Very high	About 1 in 20	Very remote	Control has very remote chance to detect
8	Very high	Loss of primary function, product inoperable	High	About 1 in 50	Remote	Control has remote chance to detect
7	High	Reduced performance, customer very dissatisfied	High	About 1 in 100	Very low	Control has very low chance to detect
6	Moderate	Comfort or convenience item inoperable	Moderate	About 1 in 500	Low	Control has low chance to detect
5	Low	Comfort item at reduced performance	Moderate	About 1 in 2,000	Moderate	Control has moderate chance to detect
4	Very low	Fit, finish or noise defect noticed by most customers	Low	About 1 in 10,000	Moderately high	Control has a moderately high chance to detect
3	Minor	Defect noticed by average customers	Low	About 1 in 100,000	High	Control has a high chance to detect
2	Very minor	Defect noticed only by discriminating customers	Remote	About 1 in 1,000,000	Very high	Control has a very high chance to detect
1	None	No discernible effect	Failure eliminated	Prevented by a proven control	Almost certain	Control will almost certainly detect, or failure prevented by design

These criteria follow the commonly used 10-point scheme from earlier AIAG FMEA editions, summarised in our own words. Failure rates are examples. Agree your own scales with your customer, document them in your FMEA procedure and use them consistently on every FMEA.

Back occurrence ratings with real data

Optional: see how scrap and defect history in your ERP can support FMEA ratings.

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How to use the scales

Find the rating row that best matches your situation and read across to the three columns. Severity rates the effect of the failure on the customer or next process, and applies to the effect, not the cause. Occurrence rates how often the cause is likely to happen given the prevention controls in place. Detection rates how well current controls would find the cause or failure before it reaches the customer. Rate the three separately and never adjust one to offset another.

Severity belongs to the effect and does not change with controls
Only design or process changes can lower severity
Occurrence needs data such as defect rates, field returns or Ppk
Detection counts current controls only

Choosing a scale for your company

The scales here are a starting point. Many companies publish their own versions, tuned to their product, such as defects per million for occurrence. Customers like the automotive OEMs publish customer specific tables as well. Keep one controlled set, give it a revision number and apply it across DFMEA and PFMEA. If you serve several customers, map your internal scale to each customer table once and record the mapping.

Examples of consistent rating

A weld that may crack and release a safety critical bracket has severity 9 or 10 even if cracking is rare. If an end-of-line test would always catch an unwelded joint, detection might be 2. A cosmetic scratch noticed by most customers rates around 4 for severity, and if the process scratches one part in a thousand, occurrence sits near 5 or 6 on a rate-based scale. Writing down why you chose each number lets the next reviewer challenge it fairly.

Common mistakes

Teams commonly lower severity after adding a control, which is wrong, rate detection on a control that does not exist yet, or rate occurrence from opinion rather than history. Another mistake is mixing scales between worksheets, which makes RPN comparisons meaningless. Review your ratings after production data accumulates and update occurrence, especially after a customer complaint or a process change.

Frequently Asked Questions

Are these the official AIAG scales?

They follow the familiar 10-point approach used in earlier AIAG editions, summarised in our own words and without copying the publication. For customer submissions use the official handbook or your customer's table.

Can I use a different scale such as 1 to 5?

Yes, if your customer allows it and you apply it consistently. Smaller scales are easier to use but give less resolution, and RPN thresholds must be rescaled accordingly.

Why can't I reduce severity with a control?

Severity describes how bad the effect is if the failure reaches the customer. Controls change how likely or how detectable the failure is, not how bad it is. Only a design or process change that alters the effect lowers severity.

How do I rate occurrence for a new process?

Use data from a similar process, pilot runs or capability studies. If none exists, rate conservatively and plan to update after production launch when real defect data becomes available.

If you want defect rates from your ERP or MES to back your occurrence ratings, Netray can show how to pull them. The scales above are free to use and export.

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