Severity, Occurance and Detection ratings seem qualitative

[raghav571]

Hello all

I am an industrial engineering student and I learned FMEA in school. My question is the ratings given to severity, occurance and detecion seems like qualitative, like something which one knows based on experience with process or product or design. Is FMEA primarily a brain storming session with persons from all levels sit together to discuss possible failures. This forum is really helping me understand things. Thanks for all your help and knowledge sharing.

Thank you
Raghav

 

[Gert Sorensen]

Re: Severity, Occurance and Detection

My question is the ratings given to severity, occurance and detecion seems like qualitative, like something which one knows based on experience with process or product or design.

In short, yes. However, there are guidelines/standards available relating to the use of FMEA in specific areas, i.e. automotive and medical devices. Keep in mind, that it is a good idea to use the ratings in the guideline, since we have a tendancy to consider the risks etc. lower than they should be, if we have been involved with the production or development of a product/process.

Is FMEA primarily a brain storming session with persons from all levels sit together to discuss possible failures.

Yes, it is basically a structured brainstorm - resulting in a list of possibilities for improvement. FMEA is a iterative process, and the benefit and the output of a FMEA session is entirely up to the team involved and the management's support of the process.

 

[raghav571]

Re: Severity, Occurance and Detection

Thank you. I am sure my learning curve is going up through this forum, Thanks for your time and guidance.

In short, yes. However, there are guidelines/standards available relating to the use of FMEA in specific areas, i.e. automotive and medical devices. Keep in mind, that it is a good idea to use the ratings in the guideline, since we have a tendancy to consider the risks etc. lower than they should be, if we have been involved with the production or development of a product/process.

Yes, it is basically a structured brainstorm - resulting in a list of possibilities for improvement. FMEA is a iterative process, and the benefit and the output of a FMEA session is entirely up to the team involved and the management's support of the process.

 

[Pazuzu - 2009]

Hello all

I am an industrial engineering student and I learned FMEA in school. My question is the ratings given to severity, occurance and detecion seems like qualitative, like something which one knows based on experience with process or product or design. Is FMEA primarily a brain storming session with persons from all levels sit together to discuss possible failures. This forum is really helping me understand things. Thanks for all your help and knowledge sharing.

Thank you
Raghav

You are correct. All the numbers generated are fairly arbitrary but should, with the right amount of analysis, be fairly accurate. Take the guidelines to heart.

The important thing to remember is to set a standard for re-evaluation of the process. Not being from the auto industry I'm not 100% positive but I think the generally accepted RPN number is 100. Anything higher (or if occurance and/or detection are high, such as >7) should prompt a reassessment of the process.

Good luck!

 

[Jim Wynne]

The important thing to remember is to set a standard for re-evaluation of the process. Not being from the auto industry I'm not 100% positive but I think the generally accepted RPN number is 100. Anything higher (or if occurance and/or detection are high, such as >7) should prompt a reassessment of the process.

No! RPN thresholds are not a good idea. RPNs should be viewed as relative, not absolute numbers, and there will be many times when something can be improved even though the RPN is less than the "trigger" number. There will also be cases where the RPN is higher than the threshold, but there will be nothing that can be done to improve the situation, and there might be several opportunities for improvement where the RPN is lower.

RPN is mostly useless, to tell the truth, and causes a lot more problems than it helps to solve.

 

[Pazuzu - 2009]

there will be many times when something can be improved even though the RPN is less than the "trigger" number. There will also be cases where the RPN is higher than the threshold, but there will be nothing that can be done to improve the situation, and there might be several opportunities for improvement where the RPN is lower.

RPN is mostly useless, to tell the truth, and causes a lot more problems than it helps to solve.

Agreed that it should be taken as reference because of the possible situations you mentioned...but is there not typically a limit as which point it should be evaluated? The threshold I'm referring to is a generic number...not an absolute. Understood that sometimes things cannot be done...and understood that sometimes you can improve it when it "seems" there is no need to...but if there is no threshold then what's the point of putting a numbering system against it?

 

[Jim Wynne]

Agreed that it should be taken as reference because of the possible situations you mentioned...but is there not typically a limit as which point it should be evaluated? The threshold I'm referring to is a generic number...not an absolute. Understood that sometimes things cannot be done...and understood that sometimes you can improve it when it "seems" there is no need to...but if there is no threshold then what's the point of putting a numbering system against it?

Again, they're relative values. This means that if you look down the RPN column on a FMEA document, and see (for example) 20, 28, 30, 31, 25 and then 108, there's something going on with the 108 that needs to be considered. But if the threshold were 100, and that last number were 90, there would be no requirement for action. At the same time, and at the risk of repetition, there might well be something in one of those first five cases (or some combination of them) that can be improved.

In most instances, people who participate in FMEAs are already aware of what they need to look out for. The RPNs just provide a method, in many instances, of documenting relative risks. The fact that there are risks doesn't mean, however, that there is always going to be a reasonable strategy to prevent bad things from happening. In my view, the FMEA process, and the document that comes out of it, serve as evidence of due diligence, and not much more.

 

[raghav571]

Thanks for all those who replied and started a discussion. Couple of them in the replies mentioned follow the GUIDELINES, are these AIAG guidelines or the guidelines of severity, occurence and detection with number 1,2,3...,10 mentioning for example hazardous with warning, hazardous with out warning in severity. Thank you all for your time.

raghav

 

[Keith Childers]

I completely agree with Jim Wynne on this one.
You have to look at the overall picture, not just the RPN.

Which of the two scenarios has the greatest need to be addressed?
1. Severity 10 x Occurrence 1 x Detection 9 = RPN 90
or
2. Severity 1 x Occurrence 10 x Detection 10 = RPN 100

In scenario 1, although the failure doesn't occur very often, it is extremely severe and has a great probability of not being detected.
Scanario 2 the failure mode happens often, is undetectable, but has no real effect.
If a RPN threshold of 100 were set, you would spend all your time trying to reduce the amount of insignificant failures while potentially harmful product was beins shipped to your customer.

 

[Pazuzu - 2009]

Thanks for all those who replied and started a discussion. Couple of them in the replies mentioned follow the GUIDELINES, are these AIAG guidelines or the guidelines of severity, occurence and detection with number 1,2,3...,10 mentioning for example hazardous with warning, hazardous with out warning in severity. Thank you all for your time.

raghav

Yes they are.