FMEA Occurrence Ranking - Based upon what?

G

g frias

#1
Hi. I work in the medical device industry and have a question about occurrence rankings in Design FMEA. In many years of using d-FMEA I've seen occurrence rankings based on both:

the probability of occurrence of the failure mode
and
the probability of occurrence of the effect of the failure (probability of harm)

The idea of the latter being that occurrence of a failure mode may or may not result in harm.

I'd love to hear your thoughts on the application. Thanks!
 
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Jim Wynne

Staff member
Admin
#2
Hi. I work in the medical device industry and have a question about occurrence rankings in Design FMEA. In many years of using d-FMEA I've seen occurrence rankings based on both:

the probability of occurrence of the failure mode
and
the probability of occurrence of the effect of the failure (probability of harm)

The idea of the latter being that occurrence of a failure mode may or may not result in harm.

I'd love to hear your thoughts on the application. Thanks!
The Occurence ranking deals with (as its name suggests) the likelihood of occurrence of the failure mode. The potential effects of the failure are addressed in the Severity ranking.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#3
If you have some need to specify the probability of the effect (a failure mode may have several different effects, each with varying probabilities of occurence and severity; and ISO 14971 the standard for risk management for medical devices is focused on the probability of harm) I would recommend listing BOTH the probability of the occurrence of the failure mode and the effects.

A word of caution, I usually encounter this when a team is trying to rationalize away the need to address a potential failure mode with a 'low probability of occurence'. Remember the intent of the DFMEA...

to further confuse the issue, in the design phase (pre-verification and validation) the probability of occurence is better stated as the liklihood of occurence of the failure mode. Probability of occurence tends to be related in most people's minds as a failure rate which is typically unknown prior to V&V testing.
 
Last edited:
I

Ingo1966

#4
Gentlemen I am very disappointed your answers are wrong !

The Occurrence is the Rating of the Probability of the CAUSE of the Failure.

In the FMEA you try to fight against the causes of the Failure if you can eliminate the cause one source of Problem is eliminated or you try to reduce the occurrence of the cause.

Remember the FMEA Form every cause has a single row that is why we focus on PREVENTION and DETECTION of the Cause.

In my FMEA Moderator Experience I see a lot of People who mix up the Effect of Failure, Cause Column --> because of that I always calibrate them to remember the Structure and Logic of a FMEA.

I think the FMEA Experts here at ELSMAR agree with me
 

Jim Wynne

Staff member
Admin
#5
Gentlemen I am very disappointed your answers are wrong !

The Occurrence is the Rating of the Probability of the CAUSE of the Failure.

In the FMEA you try to fight against the causes of the Failure if you can eliminate the cause one source of Problem is eliminated or you try to reduce the occurrence of the cause.

Remember the FMEA Form every cause has a single row that is why we focus on PREVENTION and DETECTION of the Cause.

In my FMEA Moderator Experience I see a lot of People who mix up the Effect of Failure, Cause Column --> because of that I always calibrate them to remember the Structure and Logic of a FMEA.

I think the FMEA Experts here at ELSMAR agree with me
You are correct, and it's good to point out that distinction. My response was inadequate, but I was trying to clear up the OP's misapprehension regarding the Occurrence rating having to do with the severity of effects of a given failure.
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#6
You are correct that I gave a minimal answer to the OP's specific question.

Yes - it is recommended that the probabilty of occurence be tied to the occurence of the cause. however, there are three additional factors that must be considered.

One is that the Medical Device industry (which the OP states he is in) is focused on the probabilty of harm in addition to failure and cause of failure.
A brief quote from the standard: "In cases where a hazardous situation only occurs due to a fault, the probability of a fault is not the same as
the probability of the occurrence of harm. A fault does not always result in a hazardous situation, and a hazardous situation does not always result in harm."

Secondly, while a disciplined process with knowledgable team members will result in the listing of most of the potential casues, it cannot list them all.

Third, not all causes of harm can be prevented but they can be robusted against. Notably use conditions that are distracting and simple human error.

Additionally, modern risk management is placing far less emphasis on the probability of occurence (of the cause, failure or effect/harm) and focusing on the severity of the effect and taking actions to mitigate that effect. After all probability of occurence can only be determined by actual testing (otherwise it's just a guess, subject to negotiation and human 'desire') while the severity of the effect is a matter of science and engineering.

In my 25 years doing FMEAs I simply haven't found the probability of occurence to be all that value add. I tend toward simple low medium high to avoid unproductive debate over actions that can mitigate severe effects simply because someone thinks the probability of occurence is 'low'. My approach is that when this type of debate occurs, we put rigorous tests into the verification and validation protocols and end the the debate with data...
 
G

g frias

#7
Thank you all for your comments.

It's always good to recalibrate by getting input from those outside of the same "group think" box in which you operate every day.

And you're right Bev. There is a pretty clear distinction between the "probability" and the "likelihood" of an occurrence. Likelihood would have been the better choice.

As Bev pointed out, the Medical Device industry does focus more on patient harm and that does have a tendency to confuse things from an FMEA perspective.

You were all a great help! Thanks!
 
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