In an FMEA you give each risk three scores from 1 to 10:
- Severity (S): how serious is the effect?
- Occurrence (O): how likely is it that the cause occurs?
- Detection (D): how well do you detect the problem before it reaches the customer? A high score means: hard to detect.
Based on these scores you decide which risks need attention first. There are two methods for this.
Risk Priority Number (RPN)
The RPN is the product of the three scores: RPN = S × O × D. The result lies between 1 and 1000. Companies often use a threshold, for example: above an RPN of 100 an action is mandatory.
The RPN is simple and easy to explain, but it has a well-known drawback: all three scores carry the same weight. A risk with S=10, O=2, D=5 gets the same RPN (100) as a risk with S=2, O=10, D=5. Yet the first risk, with a very serious effect, deserves much more attention.
Action Priority (AP)
The 2019 AIAG-VDA FMEA Handbook replaces the RPN with Action Priority. Instead of a multiplication, AP uses tables that place every combination of S, O and D in one of three levels:
- High (H): action is needed to reduce the risk, or you justify why the current controls are adequate.
- Medium (M): action is recommended.
- Low (L): action could be considered.
In the AP tables severity carries the most weight, then occurrence and then detection. That way a serious risk always gets priority.
Which one do you choose?
- Do you supply the automotive industry and does your customer work according to AIAG-VDA? Then AP is usually expected.
- Do you work with existing FMEAs and internal agreements based on RPN, or does your customer ask for RPN? Then RPN remains perfectly usable.
- In doubt? Check your customer’s customer specific requirements.
Both in IQARUS
In IQARUS you switch between RPN and AP with a single setting. You set the colours and thresholds for the RPN yourself, and you choose the descriptions for S, O and D from automotive or aerospace tables, or create your own. See it in the pFMEA instruction video or first read what a pFMEA is.
Ready to get started? With IQARUS you create linked pFMEAs, Process Flow Diagrams and Control Plans, with reference FMEAs that keep everything up to date. Try IQARUS free for 3 months, no account needed.
