About this lesson
DFMEA RPN and Mitigation
The product of the three scores is the Risk Priority Number (RPN). This indicates the level of technical risk due to this failure. When appropriate, action is taken to mitigate the high risks.
When to use
The RPN number is normally calculated automatically by the Design FMEA spreadsheet or form. If the RPN exceeds the business threshold number, those failure modes should be mitigated by redesign or changes to the development project plan.
The RPN is the product of the three scores. If all the scores were 1, the RPN would be 1. If all the scores were 10 ,the RPN would be 1,000. When the RPN exceeds the business threshold, normally a number near 100, the failure mode should be mitigated. The actual threshold varies by industry and many companies establish their own threshold. However, the default standard is 100.
The mitigation action will either be a change to the design or the design process. Normally, the mitigation action is focused on reducing the largest scores. After the mitigation action has been completed, the failure mode should be re-scored. If the mitigation action is a redesign, it may eliminate the failure mode, but will likely introduce new failure modes that must now be analysed and scored.
Failure modes with an RPN below the threshold can still occur – the risk is not eliminated. If the failure cause occurs, the effect will occur. However, the overall business risk is low.
Mitigating severity will require a design change. Therefore, it is one of the most difficult approaches. However, if severity is high, it will also be the most impactful. Change the design to eliminate harms and hazards to customers or users. Also, change the design so that it is more reliable, such as adding redundancy. These changes may also make the design more expensive.
Mitigate probability of cccurrence
The easiest way to mitigate probability of occurrence is to revert to a design concept that is well known and understood by the organization. This knowledge of the design, will reduce the probability that an undesirable failure mode is introduced. Often reverting to the known design concept will also allow you to use actual data rather than relying on familiarity and knowledge of the design concept or application. A frustration that can occur with this type of mitigation is that a lower score can be obtained by using old, proven technology. However, if your new product’s goal is to use new technology, you will normally have a high score in this area. Reverting to the old technology defeats the purpose of introducing new technology.
Mitigating detection requires a change to the design and development process used to create the new product. In particular, it normally means more rigorous testing that occurs earlier in the development process. This is the easiest element to change at the beginning of a project, but it becomes very expensive and adds delays if it is attempted at the end of the project. This is one of the reasons that it is important to do Design FMEAs prior to design freeze on a product.
In the pen example that has been used in previous lessons, there are three failure modes requiring mitigation. The first mitigation, adding testing, impacts the detection score of the failure of the ball not rolling. The second mitigation, changing the ball material to a different material with which there is a know history, changes the probability of occurrence score. The third mitigation, which is for the failure of the ball skipping, is a design change to tighten tolerances. After each mitigation action is complete, the failure is rescored and the results recorded in the revised ratings. The form is then maintained under revision control as part of the design documentation.
Hints & tips
- Try to mitigate the highest score first.
- Using a Poka Yoke mitigation (Mistake proofing) can essentially eliminate the failure mode and remove it from the analysis. With a Poka Yoke, the probability of occurrence and detection scores will often go to 1.
- After a product has been released into production, the form can be updated to move the revised scoring into the standard scoring. At that time the revision is now the standard. This also prepares the form for any new revision analysis that is needed because of a design change.
- Make sure the mitigation actions are actually completed before the final rescoring of the failure mode.
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