Must PFMEA Occurrence Factor be Changed Periodically?

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John C. Abnet

Teacher, sensei, kennari
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#2
Is it a must to change the occurrence rate regularly? example every month?
Good day @NPD-PFMEA .

No. The occurrence rate should be established by data / information reflecting actual conditions.

The occurrence rate (once accurately established) would not be expected to change until changes to the process are made (which then should trigger a review / update of the PFMEA).

Hope this helps.
Be well.
 
#3
There were no process changes. But there is monthly change of occurrence, that causes occurrence rate ranking changes every month. is there a rule to revise it regularly?
 
#5
Occurrence rate is being established based on monthly result of production. Result changes depending in the process reject occurrence and total qunatity. Rating can go from 1 to 3.
 

Ron Rompen

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#6
NPD: Your initial occurrence rate should be determined by previous history of similar parts and processes, and so it shows an ANTICIPATED occurrence rate.
During your PFMEA reviews you can revise the occurrence rate if there is a valid reason (i.e. data) to support it. There is no strict rule that I am aware of which specifies a frequency for review and revision (although many people, particularly auditors like to say annually); you need to determine that frequency for yourself and your company.
 

Jim Wynne

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#7
Occurrence rate is being established based on monthly result of production. Result changes depending in the process reject occurrence and total qunatity. Rating can go from 1 to 3.
I asked how the occurrence factor was originally established. Is the number of occurrences you're seeing changing anything that you're doing with regard to corrective action or acting on the process?
 

Tidge

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#8
Historically, commonly....
  • FMEA ratings are ordinal values that are only crudely related via orders of magnitude (usually I've seen "powers of ten" invoked on a scale of 1-5, but I have seen at least one set of docs scaled 1-10 that was more like "powers of 3"... I think the powers-of-3 docs had some ham radio folks developing those).
Post-control ratings typically only change when
  • A process changes, typically to improve an RPN
  • Something is discovered about the ratings (possibly pre-control ratings as well) that causes a re-assessment of the rating, typically "uh-oh" that makes an RPN worse than initially assessed.
Since the OP is implying monitoring of production output, if the rate of occurrences (of what? Maybe failure modes itself?) is consistent with the post-controls ratings I don't know why you would re-litigate any lines of a PFMEA, as observed consistency with the predicted results of process is probably not worth spending effort on.

I can imagine all sorts of circumstances in which a process exceeds expectations (usually this happens when expectations were abnormally low) but even then I wouldn't be tempted to go back and update FMEA lines... unless of course there was some personal reward... do I get a bonus for reducing RPN ratings in certain FMEA? Is this a notch on a six-sigma belt? I think it's more important to evaluate FMEA lines when the occurrence rates of failure modes are exceeding the post-control ratings. The ratings could have been a (incorrect) WAG, or the controls may be ineffective.
 
#9
If a PFMEA is created and effectively implemented through appropriately derived controls (the Control plan) shouldnt the occurrence go down? Why would anyone need to plan to adjust the PFMEA? A PFMEA doesn't exist in a vacuum, Unless you created it in one…
 

Tidge

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#10
My assumption is that the ratings (as monitored by an established control plan) being checked during production are the post-control ratings, not just implemented but also verified. If controls weren't implemented and appropriately verified (typically during a process validation), I wouldn't be quick to align a perceived improve in ratings with new and improved ratings on the post-controls side of the worksheet. blah blah correlation blah blah not causation.
 
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