# FMEA Severity, Occurrence and Detection Rating Scales > Source: https://netray.co/tools/fmea-severity-occurrence-detection-rating-scales > Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems. > Section: Free Tools ## Short answer A 1 to 10 reference for S, O and D with plain-language criteria and example failure rates. ## Category quality ## Audience - Quality engineer - Process engineer - Reliability engineer ## Tool **Kind:** worksheet **Mode:** reference **Columns:** #### Rating **Key:** rating **Type:** number #### Severity level **Key:** sev_label **Type:** text #### Severity criteria (summary) **Key:** sev_desc **Type:** text #### Occurrence level **Key:** occ_label **Type:** text #### Example failure rate **Key:** occ_rate **Type:** text #### Detection level **Key:** det_label **Type:** text #### Detection criteria (summary) **Key:** det_desc **Type:** text **Rows:** **Rating:** 10 **Sev label:** Hazardous, no warning **Sev desc:** Safety or regulatory effect with no warning **Occ label:** Very high **Occ rate:** 1 in 10 or more **Det label:** Almost impossible **Det desc:** No known control can detect the failure **Rating:** 9 **Sev label:** Hazardous, with warning **Sev desc:** Safety or regulatory effect with warning **Occ label:** Very high **Occ rate:** About 1 in 20 **Det label:** Very remote **Det desc:** Control has very remote chance to detect **Rating:** 8 **Sev label:** Very high **Sev desc:** Loss of primary function, product inoperable **Occ label:** High **Occ rate:** About 1 in 50 **Det label:** Remote **Det desc:** Control has remote chance to detect **Rating:** 7 **Sev label:** High **Sev desc:** Reduced performance, customer very dissatisfied **Occ label:** High **Occ rate:** About 1 in 100 **Det label:** Very low **Det desc:** Control has very low chance to detect **Rating:** 6 **Sev label:** Moderate **Sev desc:** Comfort or convenience item inoperable **Occ label:** Moderate **Occ rate:** About 1 in 500 **Det label:** Low **Det desc:** Control has low chance to detect **Rating:** 5 **Sev label:** Low **Sev desc:** Comfort item at reduced performance **Occ label:** Moderate **Occ rate:** About 1 in 2,000 **Det label:** Moderate **Det desc:** Control has moderate chance to detect **Rating:** 4 **Sev label:** Very low **Sev desc:** Fit, finish or noise defect noticed by most customers **Occ label:** Low **Occ rate:** About 1 in 10,000 **Det label:** Moderately high **Det desc:** Control has a moderately high chance to detect **Rating:** 3 **Sev label:** Minor **Sev desc:** Defect noticed by average customers **Occ label:** Low **Occ rate:** About 1 in 100,000 **Det label:** High **Det desc:** Control has a high chance to detect **Rating:** 2 **Sev label:** Very minor **Sev desc:** Defect noticed only by discriminating customers **Occ label:** Remote **Occ rate:** About 1 in 1,000,000 **Det label:** Very high **Det desc:** Control has a very high chance to detect **Rating:** 1 **Sev label:** None **Sev desc:** No discernible effect **Occ label:** Failure eliminated **Occ rate:** Prevented by a proven control **Det label:** Almost certain **Det desc:** Control will almost certainly detect, or failure prevented by design **Notes:** 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. ## Content **Intro:** 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. **Sections:** #### How to use the scales **Body:** 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. **Bullets:** - 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 **Body:** 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 **Body:** 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 **Body:** 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. **Cta:** 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. **Faq:** #### Are these the official AIAG scales? **Answer:** 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? **Answer:** 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? **Answer:** 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? **Answer:** 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. ## Search terms fmea rating scales, fmea severity scale, fmea occurrence rating table, fmea detection rating table, fmea ranking criteria, severity occurrence detection table, fmea rating 1 to 10, fmea ranking table pdf --- Talk to an engineer: https://netray.co/contact?intent=erp_ai | Machine catalog: https://netray.co/agent/catalog.json