The risk priority number, or RPN, multiplies the three scores together: RPN = S × O × D. Because each score runs 1 to 10, the RPN runs from 1 to 1,000. It is a single figure you can sort by, which makes it a convenient first pass for deciding where to spend improvement effort when you have forty rows and time for five.
Treat the RPN as a ranking tool, not a measurement. An RPN of 200 is not "twice as risky" as an RPN of 100, because the three scores are judgments on ordinal scales. Two rows with the same RPN can describe very different risks: 10 × 2 × 10 and 5 × 8 × 5 both equal 200, but one is a rare, undetectable harm and the other is a frequent, moderately caught nuisance.
Sort the rows from highest RPN to lowest, then read the top of the list with the individual S, O, and D values beside each RPN. The number gets your attention; the three scores tell you why.
Standard Procedure
- 1Multiply severity, occurrence, and detection for each row and record the RPN.
- 2Sort the rows from highest to lowest RPN.
- 3For the top rows, note which of S, O, or D is driving the number.
- 4Check any rows with equal RPNs and compare their individual scores before treating them as equivalent.
Lesson Summary
- ✓RPN is a product of three scores and runs from 1 to 1,000.
- ✓Use it to order the work, not to claim one risk is a precise multiple of another.