I want to inject a note of caution regarding terminology:

First, for harm to happen, a hazardous situation must arise, which would be probability P1.

P1 would then be **multiplied **by P2, which would be the probability that the hazardous situation leads to actual harm, hence:

P (probability of occurence of harm) = P1 (probability that the hazardous situation occurs) * P2 (probability for harm in the specific situation)

We (in the industry of medical device risk management) use the term "multiply" liberally, and (almost always) only as shorthand to indicate that there are "multiple" factors that contribute to an occurrence of HARM. It is

*common *that an actual "multiplication" of P1 and P2

*ratings *(along with

*ratings *of Severity) appear in Risk Analyses, but those

*ratings *are established by policy and do not typically represent a precise measurement with well-established uncertainties.

For medical devices, it would be unusual to have precise measurements for all the circumstances where we

*need *assessments of P1 and P2, and practically it would be futile to try to establish them given the wide variability in humans (and medical device design possibilities). Typically there are simply broad (integer)

*ratings *of P1, P2 established such that ordinal values of the ratings offers the ability to make a quick, broad assessment of the relative value of the probabilities and is not meant to be a precise estimate. It is common to use five ordinal ratings for P1 and P2 (and Severity), I believe this is the practice because "only three" implies a little too much simplicity (

*a la* Goldilocks and the Three Bears) and "more than five" invites too much effort to categorize by

*degree*.

For example, depending on the nature of the device, and the context of use, ratings can be established as appropriate...

Example: "Compromised Graft Integrity" (this Hazardous situation is **defined in the SOP**) and in Risk analysis document **the P1 value of this Hazardous Situation is 0.01%, 0.05 and 0.10%** in different lines.

In my experience, with my personal biases, I can see a "powers of ten" difference between 0.01% and 0.10%... and with more direct experience I might be swayed that there was a meaningful enough difference between 0.01% and 0.05% or 0.05% and 0.10% using a different power scale (than 'powers of 10') so understand I am not passing judgement on these divisions, just on the implied precision.