Home>Scoring Criteria

Severity Rating (S)

Severity assesses the impact on the end user or system when a failure mode occurs. Once established, the severity rating typically does not change due to improvement actions (unless the design itself is modified).

Rating Impact Level DFMEA Description PFMEA Description
10 Hazardous without warning Affects safe vehicle operation without warning May endanger operator without warning
9 Hazardous with warning Affects safe vehicle operation with warning May endanger operator with warning
8 Very High Vehicle/system inoperable, loss of primary function 100% product scrapped, line shutdown for repair
7 High Vehicle operable but with significantly degraded performance Product to be sorted, portion scrapped
6 Moderate Vehicle operable but comfort/convenience reduced Product reworkable (off-line)
5 Low Vehicle operable, minor performance impact Product reworkable (in-line)
4 Very Low Minor effect noticed by most customers Sorting required, no scrap
3 Minor Minor effect noticed by some customers Process adjustment, no line stoppage
2 Very Minor Effect noticed only by discriminating customers Appearance defect, limited products affected
1 None No discernible effect No effect on process

Occurrence Rating (O)

Occurrence assesses the likelihood of a failure cause occurring. The rating should be based on the effectiveness of existing prevention controls.

Rating Likelihood Prevention Control Status Reference Frequency
10 Extremely High No prevention control or completely ineffective 100 or more per thousand
9 Very High Prevention control nearly ineffective 50 per thousand
8 High Prevention control very poor effectiveness 20 per thousand
7 Moderately High Prevention control poor effectiveness 10 per thousand
6 Moderate Prevention control limited effectiveness 2 per thousand
5 Moderately Low Prevention control some effectiveness 0.5 per thousand
4 Low Prevention control effective 0.1 per thousand
3 Very Low Prevention control very effective 0.01 per thousand
2 Extremely Low Prevention control highly effective 0.001 per thousand or less
1 Nearly Impossible Failure eliminated through prevention control Eliminated through error-proofing

Detection Rating (D)

Detection assesses the ability of existing detection controls to discover the failure before the product is delivered to the customer.

Rating Detection Ability Detection Control Description
10 Almost Impossible No detection control or detection is not possible
9 Very Remote Detection control unlikely to detect the failure
8 Remote Low chance detection control will detect the failure
7 Very Low Very low chance detection control will detect the failure
6 Low Some chance detection control will detect the failure
5 Moderate Moderate chance detection control will detect the failure
4 Moderately High Good chance detection control will detect the failure
3 High High chance detection control will detect the failure
2 Very High Detection control almost certain to detect the failure
1 Almost Certain Automatic error-proof detection; failure cannot escape

Action Priority (AP)

The new AIAG-VDA FMEA Handbook introduces Action Priority (AP) to replace traditional RPN ranking. AP is determined as three levels based on the S-O-D score combination via lookup tables:

High (H)

Action required
Current controls are inadequate. Improvement actions must be identified to reduce risk.

Medium (M)

Action recommended
Improvement actions should be considered to reduce risk.

Low (L)

Acceptable
Current controls are adequate. Further improvement is optional.

AP vs. RPN Comparison

Aspect RPN (Traditional) AP (New Method)
Calculation S x O x D = RPN value S-O-D combination lookup table
Result Format Continuous value from 1 to 1000 Three levels: H/M/L
Decision Method Rank by numerical value Action requirements determined by level
Main Advantage Simple and intuitive Avoids mathematical misleading, emphasizes Severity weight
Main Limitation Same RPN may represent completely different risks Requires AP lookup table reference
RPN Limitation Example: S=10, O=1, D=1 gives RPN=10 (very low), but Severity=10 means a safety impact! Meanwhile, S=3, O=4, D=4 gives RPN=48 (higher), but the actual risk is far lower than the first case. The AP method addresses this by giving higher weight to Severity.

Traditional RPN Calculation Reference

Although the new handbook recommends AP, many organizations still use RPN as a supplementary reference during the transition:

RPN = S x O x D

Range: 1 to 1000

RPN 200 or higher

High Risk - Action required

RPN 80 to 199

Medium Risk - Improvement recommended

RPN below 80

Low Risk - Acceptable