The next time you need to think about how much risk is appropriate in a safety-critical system, try these existing approaches on for size instead of making up something on your own:
ALARP: "As Low As Reasonably Practicable" Some risks are acceptable. Some are unacceptable. Some are worth taking in exchange for benefit, but if that is done the risk must be reduced to be ALARP.
GAMAB: "Globalement Au Moins Aussi Bon" Offer a level of risk at least as good as the risk offered by an equivalent existing system. (i.e., no more dangerous than what we have already for a similar function)
MEM: "Minimum Endogenous Mortality" The technical system must not create a significant risk compared to globally existing risks. For example, this should cause a minimal increase in overall death rates compared to the existing population death rates.
MGS: "Mindestens Gleiche Sicherheit" (At least the same level of safety) Deviations from accepted practices must be supported by an explicit safety argument showing at least the same level of safety. This is more about waivers than whole-system evaluation.
NMAU: "Nicht Mehr Als Unvermeidbar" (Not more than unavoidable) Assuming there is a public benefit to the operation of the system, hazards should be avoided by reasonable safety measures implemented with reasonable cost.
Each of these approaches has pros and cons. The above terms were paraphrased from this nice discussion:
Kron, On the evaluation of risk acceptance principles,
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.455.4506&rep=rep1&type=pdf
There is an interesting set of slides that covers similar ground here, and works some examples. In particular the graphs involving whether risks are taken voluntarily for different scenarios is thought provoking:
http://agse3.informatik.uni-kl.de/teaching/suze/ws2014/material/folien/SRES_03_Risk_Acceptance.pdf
In general, if you want to dig deeper into this area, a search on
gamab mem alarp
will bring up a number of hits
Also note that legal and other types of considerations exist, especially regarding product liability.
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