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Why is the FMEA RPN = S*O*D?


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Related Topic Tags
detection ranking (fmea), fmea - failure modes and effects analysis, fmea rankings, formula (mathematics/statistics), occurrence ranking (fmea), rpn (fmea risk priority number), severity ranking (fmea)
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  Post Number #1  
Old 3rd October 2012, 09:11 AM
qq130230416

 
 
Total Posts: 5
Please Help! 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?

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  Post Number #2  
Old 3rd October 2012, 09:32 AM
Marc's Avatar
Marc

 
 
Total Posts: 25,329
Re: Why is the FMEA RPN = S*O*D?

For starters, see: FMEA Risk Factors
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  Post Number #3  
Old 3rd October 2012, 09:54 AM
Statistical Steven's Avatar
Statistical Steven

 
 
Total Posts: 1,064
Re: Why is the FMEA RPN = S*O*D?

Quote:
In Reply to Parent Post by qq130230416 View Post

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  
Old 3rd October 2012, 12:35 PM
antoine.dias's Avatar
antoine.dias

 
 
Total Posts: 512
Re: Why is the FMEA RPN = S*O*D?

Quote:
In Reply to Parent Post by Statistical Steven View Post

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  
Old 3rd October 2012, 02:43 PM
Statistical Steven's Avatar
Statistical Steven

 
 
Total Posts: 1,064
Re: Why is the FMEA RPN = S*O*D?

Quote:
In Reply to Parent Post by antoine.dias View Post

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  
Old 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 View Post

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  
Old 4th October 2012, 10:26 AM
Jim Wynne's Avatar
Jim Wynne

 
 
Total Posts: 14,155
Re: Why is the FMEA RPN = S*O*D?

Quote:
In Reply to Parent Post by qq130230416 View Post

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  
Old 4th October 2012, 01:34 PM
Bev D's Avatar
Bev D

 
 
Total Posts: 3,398
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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