I don't think you have any real scientific basis for those numbers. I doubt most people can reduce their risk of infection to 50% unless they take extreme and unsustainable steps. Please state your assumptions and show your math.
My assumption is that a sizeable fraction of people either:
* Already got it
* Don’t take hand washing seriously or do it wrong
* Are bad at avoiding touching their face
* Unnecessarily touch surfaces when out (i.e. door with hand, rather than sleeve/bag, whatever)
* Don’t practice distancing well
* Are in a job where they must interact with the public
If you do all of the above correctly, my guess is you can raise your odds of avoiding from 30% to 50-60%.
It’s not based on any math, but I don’t think there’s any definite math to base it on. I could be wrong in either direction.
Nonetheless I think it’s sufficient to argue that it is not inevitable that and individual will get it and everyone should not even bother to avoid it.
Is that the normal way of phrasing risk? My expectation would be that there is a 30% chance of not getting it. Am I misreading your comment or is my definition of risk somehow backward?