The Elsmar Cove Business Systems and Standards Discussion Forums Why is the FMEA RPN = S*O*D?
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# Why is the FMEA RPN = S*O*D?

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Post Number #1
3rd October 2012, 09:11 AM
 qq130230416 Total Posts: 5
Why is the FMEA RPN = S*O*D?

Dear all
i have a question, why RPN is S*O*D? what is meaning for this formula?

Post Number #2
3rd October 2012, 09:32 AM
 Marc Total Posts: 25,731
Re: Why is the FMEA RPN = S*O*D?

For starters, see: FMEA Risk Factors
Post Number #3
3rd October 2012, 09:54 AM
 Statistical Steven Total Posts: 1,079
Re: Why is the FMEA RPN = S*O*D?

Quote:
 In Reply to Parent Post by qq130230416 Dear all i have a question, why RPN is S*O*D? what is meaning for this formula?
S = Severity
O = Occurrence
D = Detection

Severity is how "bad" the injury would be if there was a failure (1=none, 10=death)

Occurrence is how likely is the defect to occur (1=unlikely, 10=often)

Detection is how likely are your controls to detecting the defect should it occur (1 = Always detected, 10 = never detected)

You rate each failure mode on a rating of 1 to 10 for each of the three. Multiply the ratings to get the RPN for the failure mode.

A flaw with this rankin is that a failure mode with very low severity, high occurrence with low detection would score an RPN of 100, while a failure mode with high severity, low occurence and low detection would also score a 100. I favor a rating system that weights the severity more than detection.

 Thank You to Statistical Steven for your informative Post and/or Attachment!
Post Number #4
3rd October 2012, 12:35 PM
 antoine.dias Total Posts: 514
Re: Why is the FMEA RPN = S*O*D?

Quote:
 In Reply to Parent Post by Statistical Steven A flaw with this rankin is that a failure mode with very low severity, high occurrence with low detection would score an RPN of 100, while a failure mode with high severity, low occurence and low detection would also score a 100. I favor a rating system that weights the severity more than detection.
In the new version of the FMEA manual (AIAG) they present SOD as an alternative way to the RPN ranking
Example :
S= 9 / O= 4 / D= 5 would mean RPN= 180 but SOD = 945
S= 4 / O= 9 / D= 5 would mean RPN= 180 but SOD = 495
S= 5 / O= 4 / d= 9 would mean RPN =180 but SOD = 549

In the past years SOD is applied in several companies I visit, just for the reason you stated.

best regards,

Antoine
 Thank You to antoine.dias for your informative Post and/or Attachment!
Post Number #5
3rd October 2012, 02:43 PM
 Statistical Steven Total Posts: 1,079
Re: Why is the FMEA RPN = S*O*D?

Quote:
 In Reply to Parent Post by antoine.dias In the new version of the FMEA manual (AIAG) they present SOD as an alternative way to the RPN ranking Example : S= 9 / O= 4 / D= 5 would mean RPN= 180 but SOD = 945 S= 4 / O= 9 / D= 5 would mean RPN= 180 but SOD = 495 S= 5 / O= 4 / d= 9 would mean RPN =180 but SOD = 549 In the past years SOD is applied in several companies I visit, just for the reason you stated. best regards, Antoine
My only concern is how does the AIAG deal with a situation of the following?
S= 9 / O= 1 / D= 1 with RPN= 9 but SOD = 911
S= 8 / O= 8 / D= 8 with RPN=512 but SOD = 888
Post Number #6
4th October 2012, 10:12 AM
 qq130230416 Total Posts: 5
Re: Why is the FMEA RPN = S*O*D?

Quote:
 In Reply to Parent Post by Statistical Steven S = Severity O = Occurrence D = Detection Severity is how "bad" the injury would be if there was a failure (1=none, 10=death) Occurrence is how likely is the defect to occur (1=unlikely, 10=often) Detection is how likely are your controls to detecting the defect should it occur (1 = Always detected, 10 = never detected) You rate each failure mode on a rating of 1 to 10 for each of the three. Multiply the ratings to get the RPN for the failure mode. A flaw with this rankin is that a failure mode with very low severity, high occurrence with low detection would score an RPN of 100, while a failure mode with high severity, low occurence and low detection would also score a 100. I favor a rating system that weights the severity more than detection.
thank you so much.
i am sorry. my english is poor so i can not express clearly.
i mean why not RPN≠S*O or S*D? what does S*O*D mean in statistics?
Post Number #7
4th October 2012, 10:26 AM
 Jim Wynne Total Posts: 14,160
Re: Why is the FMEA RPN = S*O*D?

Quote:
 In Reply to Parent Post by qq130230416 thank you so much. i am sorry. my english is poor so i can not express clearly. i mean why not RPN≠S*O or S*D? what does S*O*D mean in statistics?
They mean nothing in statistics--they are relative values, not absolute, and the scaling is mostly arbitrary. In other words, the idea is that there is a consistent rating scale, and it should be usable in terms of deciding which are the most important things to improve or be aware of.
 Thanks to Jim Wynne for your informative Post and/or Attachment!
Post Number #8
4th October 2012, 01:34 PM
 Bev D Total Posts: 3,488
Re: Why is the FMEA RPN = S*O*D?

Jim has it correct - these are really arbitrary rankings. They have no statistical validity or mathematical validity or science validity. There was a desire to try to quantify ratings ratehr than use a qualitative ranking.

Some people do use S*O
I haven't heard of anyone using S*D

A 'recent' development is to use a matrix of Severity by Occurrence that isn't mathematically derived.

Another development primarily from the medical device community is to use just the severity and allow teh testign to determine the occurence rate.
 Thank You to Bev D for your informative Post and/or Attachment!

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