Focus video player for keyboard shortcuts
Auto
  • 720p
  • 540p
  • 360p
1.00x
  • 0.50x
  • 0.75x
  • 1.00x
  • 1.25x
  • 1.50x
  • 1.75x
  • 2.00x
cc

We hope you enjoyed this lesson.

Get the Failure Mode and Effects Analysis course for more great video tutorials.

Start free trial

Cool lesson, huh? Share it with your friends

About this lesson

The severity, occurrence, and detection ratings are combined to create a Risk Priority Number (RPN). When that RPN exceeds the organization's risk threshold for Design FMEAs, a mitigation strategy is developed and implemented to lower the scores until the RPN is acceptable.

Exercise files

Download this lesson’s related exercise files.

DFMEA RPN and Mitigation
61.3 KB DFMEA RPN and Mitigation - Solution
442.6 KB

Quick reference

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.

Instructions

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.

Mitigate severity

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.

Mitigate detection

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.

Example

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.
Login to download
  • 00:04 Hi, I'm Ray Sheen.
  • 00:06 Well, we've done the scoring, so now it's time to move to step six and
  • 00:10 the RPN mitigation.
  • 00:13 This topic starts with the Risk Priority Number, or RPN.
  • 00:17 The RPN is the product of severity, occurrence, and detection scores.
  • 00:21 Now, those can range from 1 to 10.
  • 00:24 So if all 3 were 1, the RPN would be a 1, but if all 3 are 10, the RPN is 1,000.
  • 00:29 Frankly I've never seen either of those extremes,
  • 00:33 the number has always been somewhere between those two.
  • 00:36 Each organization sets a threshold of the value of RPN based upon risk sensitivity.
  • 00:41 If the RPN for
  • 00:42 a failure mode exceeds the threshold then a mitigation action is required.
  • 00:46 If the RPN is less than the threshold then no mitigation is needed.
  • 00:50 The threshold varies from industry to industry and even company to company.
  • 00:54 However, a common threshold value is 100 and that is what I will be using for
  • 00:58 the rest of this course.
  • 01:00 Of course, if your company has a different number, use it.
  • 01:03 Some organizations require a mitigation if the criticality scores is high.
  • 01:07 Recall from our lesson on criticality that for
  • 01:10 that value we use only the severity and probability scores.
  • 01:14 One other point about mitigation, the results of a mitigation may eliminate some
  • 01:18 failure modes but it may also create new failure modes that must then be analyzed.
  • 01:23 Now just to be clear, if the RPN is below the threshold,
  • 01:26 that does not mean that there is no risk.
  • 01:28 It can still occur, and if it does, the failure mode effect will occur.
  • 01:32 However, because of the scores, the overall business technical risk is low.
  • 01:38 Let's look at the mitigation strategies.
  • 01:40 Normally, you try to mitigate the highest scores.
  • 01:42 But ultimately, you mitigate whatever you can.
  • 01:45 I'll start with strategies for mitigating the severity score.
  • 01:49 This is often the most difficult to mitigate because many times,
  • 01:52 the score is inherent in the function of the product.
  • 01:55 Automobiles can crash and kill people, so any failure in the drive train, or
  • 02:00 control control of a car will likely be very high.
  • 02:03 Well, the only way to keep the car from crashing is if it never moves, but
  • 02:08 that then defeats the purpose of the car.
  • 02:10 However once past the danger of injury,
  • 02:13 there are things that can be done to lower the scores by creating redundant or
  • 02:16 backup systems, so that the operability is not impacted by a failure.
  • 02:21 Many times, to be able to lower a severity risk, a change will need to made
  • 02:25 to the product specification or to the fundamental design approach.
  • 02:29 This is one of the reasons that you wanna do the Design FMEA before design freeze so
  • 02:33 that change to the design approach is still possible.
  • 02:36 In addition to redundancy, to reduce high scores,
  • 02:38 you may also need to add safety attributes to the product,
  • 02:42 such as putting fuses in an electrical circuit to prevent a shock hazard.
  • 02:46 This could lower the score from a 10 to a 7.
  • 02:49 Okay, now let's look at mitigating the probability of occurrence.
  • 02:52 Now this score was based upon one of three scales.
  • 02:56 Actual data of occurrence, the design history of similar products and
  • 03:00 the knowledge of the customer application.
  • 03:02 We said that you should use data if you have it,
  • 03:04 otherwise the worst case of the other two.
  • 03:07 So the obvious change would be the use of different technology or
  • 03:10 technical approach, where you have data that would provide a lower score.
  • 03:15 Rather than trying to reinvent the wheel, use what already works.
  • 03:19 A known good system or design.
  • 03:21 If you don't have good data, then use an approach that you have experience with.
  • 03:25 These would be materials and components that are well-established.
  • 03:29 Because of your experience with them, there is little uncertainty.
  • 03:32 But here is the problem with that approach.
  • 03:35 By continuing to use the same old designs,
  • 03:37 you don't get the competitive advantage of new products, materials or systems.
  • 03:42 This is a tradeoff.
  • 03:43 The new technology may have major market advantages, but
  • 03:47 it is a higher technical risk.
  • 03:49 This could also mean that you're using a more expensive but tried and
  • 03:53 true technology rather than the less expensive but uncertain technology.
  • 03:58 These are business decisions.
  • 03:59 A decision to use the new technology may be the right business decision, but
  • 04:04 management also needs to recognize and understand the technical risks involved,
  • 04:09 and the Design FMEA can help to explain those risks.
  • 04:12 Another way to improve the probability of occurrence is to spend more time
  • 04:16 with the customer and the application.
  • 04:18 A better understanding will reduce the likelihood that the design may fail on
  • 04:22 the application.
  • 04:23 Understanding the use and potential misuse of the product
  • 04:26 will help the design team make better design decisions.
  • 04:30 The third type of mitigation is to mitigate the detection score, and
  • 04:33 that will mean changing the product development project plan.
  • 04:36 This is usually the easiest mitigation to implement, provided the Design FMEA
  • 04:41 is done early enough in the development process to influence the testing and
  • 04:45 analysis that is done.
  • 04:47 If the Design FMEA is done late in the development, well, then the mitigation
  • 04:50 will require additional retests and new analysis and this will likely create
  • 04:55 delays and overruns in the product development projects.
  • 04:58 By just reviewing the scoring criteria, you can see that early and
  • 05:01 robust testing of the different failure modes is a key to lowering the score.
  • 05:06 These tests will uncover design weaknesses and give the designers the data they
  • 05:09 need to enhance the design so that the failure mode is less likely to occur.
  • 05:14 It will provide the data needed to model the performance and
  • 05:17 accurately predict how the system will work under different conditions.
  • 05:22 This early testing will also allow time to add features that prevent failures or
  • 05:26 provide an early warning of the failure mode.
  • 05:28 These Poka Yoke mistake proofing features will lower both the detection score and
  • 05:33 in some cases the severity score by providing warnings.
  • 05:37 So I've completed the scoring of the ball for the ballpoint pen.
  • 05:41 As you can see, three of the failure modes scored above the 100 threshold, so
  • 05:45 they required mitigation.
  • 05:47 For the failure where the ball stops rolling because the ink has drained out
  • 05:51 and acted as a glue holding it in place, we had a score of 105.
  • 05:55 In this case,
  • 05:56 I mitigated the detection by adding a degradation test to the project plan
  • 06:01 which will provide the data to determine the condition when this is likely.
  • 06:05 So the different ink can be used if needed.
  • 06:08 Notice on the form, it's not enough to think of a mitigation,
  • 06:11 you must actually perform that mitigation and then rescore the failure mode.
  • 06:16 The next failure mode is the ball corrosion.
  • 06:18 In this case, the mitigation impacted the probability of occurrence.
  • 06:22 By changing to a different material, stainless steel,
  • 06:25 the data on corrosion allows the score to drop from a 5 to a 3.
  • 06:29 The RPN is now on a safe zone below a 100, but it marked all RPNs in the 75 to
  • 06:34 100 range as yellow, meaning that they need to be watched in production.
  • 06:38 The third failure requiring mitigation was the ball is skipping.
  • 06:42 This mitigation was addressed at the severity level by tightening the design
  • 06:46 tolerance so that the product would perform well when first delivered.
  • 06:51 It may still get some problems over time, but
  • 06:53 the initial performance is improved and the score was lowered to the safe zone.
  • 06:57 Now that everything is mitigated, I can perform step seven of our process and
  • 07:03 put this document under revision control, and
  • 07:05 maintain it with the rest of the design documentation.
  • 07:08 The ultimate goal of the Design FMEA
  • 07:12 was to identify technical risks in the product and appropriately resolve it.
  • 07:17 With the mitigation done, the goal has been achieved.

Lesson notes are only available for subscribers.

PMI, PMP, CAPM and PMBOK are registered marks of the Project Management Institute, Inc.

Gift this course