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Doing both a top-down and a bottom-up risk assessment - How to combine


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If conducting both a top-down (e.g. FTA, PHA) and a bottom-up (e.g. FMEA) risk assessment for a design of a medical product how does one combine the results? Is their a normal way? I assume they should be done independently from each other but one concern is in conducting both assessments there could be lots of duplication and the possibility of one documents contradicting the other.

Thanks for any input.
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Ronen E

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Staff member
I'm not sure how you could end up with contradictions, but I certainly agree that you're up for a lot of duplication.

If you're looking to follow ISO 14971, it's basis is identifying Hazards and Hazardous Situations (see definitions in the standard). No matter what analysis method(s) you use, these should be the outputs to look for. Once done, combine all to one list, then proceed according to the procedure the standard specifies.


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Thanks Ronen. If I was to take a scalpel as an example to illustrate by questions/concerns. Maybe my top-down PHA identified the need for the scalpel to have a certain pattern of grip on the shaft to prevent to prevent a doctor's hand from slipping and injuring the patient. In the PHA I establish a probability and severity of harm occurring prior to the grip being added. The grip (risk control) then brings the probability of harm down to a post-imitator level.

Later on in the project a DFMEA is conducted on the scalpel. When looking at the shaft the failure mode, "grip insufficient" is identified. Assume the DFMEA team does not know this has been identified as a risk control in the PHA as there is no formal link between the two. The grip pattern is reviewed by the team and it is decided it is insufficient due to the fact that the doctor may be wearing gloves. Hence the design is changed. The probability of harm identified by the DFMEA team pre the change is higher than the probability in the PHA as the PHA didn't consider the use of gloves. Hence you can get a contradiction.

So I guess I was wondering if there is a way to ensure things like this don't happen? It also gets complicated because do you go and update the PHA with the grip pattern that was identified in the FMEA? Also, maybe the PHA didn't call for a grip and put in something else to prevent slipping but then the FMEA identified the need for a grip so you have multiple risk control which is fine but how do you then combine these things to look at residual risk.

It seems like it would get complicated quite quickly...

Ronen E

Problem Solver
Staff member
I see it as refinement rather than contradiction. The Risk Management is a live process and the Risk Management File (singular) is a live document. All these processes need to feed into the RMF as revisions. That would also ensure that different teams working at different stages stay informed with the work of others, because everyone would be working off the same document.

The probability estimates in the RMF should always reflect the best knowledge existing at the time of making them. Further, if this is managed as formal revisions of the RMF, there should be no need to update backwards. It should be clear enough what estimates were issued at what time/stage, and why, and it should also be clear why and how they were subsequently updated.

BTW I think the example you gave is not the best. What you described as a DFMEA entry seems to me like a reworded HA entry. In my understanding FMEA is about component failure, and that example involved no such failure.


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Thanks again. That makes sense. In regards to the FMEA, my understanding is that one use of this document is to capture if the component design is inadequate to meet its function. Hence even though its not a component failure in the strict sense (its just badly designed and specified) the FMEA can still capture this. Although if this is incorrect please let me know. I think it gets a little messy as different people seem to have different rules for FMEAs.

Ronen E

Problem Solver
Staff member
different people seem to have different rules for FMEAs.
That's why I dislike FMEAs in general and I try to avoid them in the ISO 14971 context, except maybe for complex devices/systems where the consequences of individual component failure are otherwise quite difficult to capture.


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You should have a hazard analysis document that lists hazardous situations and risk levels determined from both the top down and bottom up approaches. Regardless of how you do it, all documents need to be linked.
@indubioush Could you expand on this approach of having a hazard analysis document that contains risk levels determined from both the top down and bottom up approach? I thought the hazard analysis document itself is usually the top down approach. Thanks.


Quite Involved in Discussions
The hazard analysis could be considered a top-down approach depending on how you determine foreseeable hazardous situations. The typical top-down approach is the fault tree analysis, but not every company does this. If you do both FTA and FMEA, you would have a hazard analysis document that brings the info from these sources together so you have one document that lists all hazardous situations and the risk level assigned to them. If you only have a hazard analysis and fmea documents, your hazard analysis should still list all hazardous situations and their associated risk levels. There should be linkages to the FMEA document so that tracing all sequences of events and probabilities for each hazardous situation is possible, and thereby, the assigned risk level in the hazard analysis can be verified.
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