The Risk Priority Number is the product of three ratings — Severity, Occurrence and Detection — each assigned on a 1 to 10 scale. The product is then used to rank failure modes and, in most implementations, compared against a threshold to decide which receive action.

Both operations assume the ratings carry interval information. They do not.

Ordinal ratings under multiplication

Severity, Occurrence and Detection are ordinal: the ratings order failure modes without fixing the distance between adjacent levels. On any severity scale in common use the upper levels denote safety and regulatory consequence while the lower levels denote cosmetic effect, and the interval between 9 and 10 is not equal to the interval between 1 and 2. Stevens (1946) sets out which operations are admissible on which scale types; multiplication is not among those permitted on ordinal data.

The consequences are measurable rather than theoretical.

Resolution. The 1000 combinations available on three 1–10 scales produce 120 distinct RPN values. RPN 60 alone is produced by 24 different combinations. The statistic maps 1000 assessment states onto 120 outputs, and the combinations mapped together are not equivalent in risk.

Ordering. The maximum RPN attainable at Severity 2 is 200. The minimum RPN attainable at Severity 10 is 10. A cosmetic failure occurring frequently and escaping detection therefore outranks a fatal failure that is rare and detectable, by a factor of twenty.

Threshold behavior. Under the widely used rule that action is required above RPN 100, 23 of the 100 severity-10 combinations fall below the threshold and are not flagged. The proportion is a property of the arithmetic, not of any particular process.

Action Priority

The AIAG-VDA FMEA Handbook, 1st Edition (2019) removed RPN thresholds in favor of Action Priority: a classification of High, Medium or Low read from a lookup table indexed on the combination of Severity, Occurrence and Detection rather than on their product.

Because the table is indexed rather than computed, it can encode rules that a product cannot express. A failure mode rated Severity 9 or 10 with Occurrence 4 or above is classified High irrespective of Detection. Under RPN the same failure mode with Detection 2 has its score divided by five and may fall below any threshold set on the product.

The ratings themselves are unchanged. Assignment of Severity, Occurrence and Detection remains the substantive work of an FMEA, and Action Priority alters only how the three assigned values are combined into a priority.

Practical notes

Screening an existing FMEA. Sorting an existing worksheet by Severity rather than by RPN identifies rows exposed to the threshold behavior above. Rows at Severity 9 or 10 falling below the action threshold are the affected set.

Retaining RPN. Customer specifications and existing quality agreements frequently require RPN, and it can be reported alongside Action Priority without conflict. The two are computed from the same three ratings; where they disagree, the disagreement is attributable to the threshold behavior rather than to a difference in assessment.

SVEND's FMEA calculator reports RPN and AIAG-VDA Action Priority for the same inputs.

References

Stevens, S. S. (1946). On the theory of scales of measurement. Science 103(2684), 677–680.

AIAG-VDA (2019). Failure Mode and Effects Analysis: FMEA Handbook, 1st Edition. Automotive Industry Action Group and Verband der Automobilindustrie.