With A & B iid normal variables, if you take someone who is 1 in a billion at A+B, then in the median case they will be 1 in 90,000 at A. Then if you take someone who is 1 in 90,000 at A and give them the median level of B, they will be 1 in 750 at A+B.
(You can get to rarer levels by reintroducing some of the variation rather than taking the median case twice.)
Oh, you’re right.
With A & B iid normal variables, if you take someone who is 1 in a billion at A+B, then in the median case they will be 1 in 90,000 at A. Then if you take someone who is 1 in 90,000 at A and give them the median level of B, they will be 1 in 750 at A+B.
(You can get to rarer levels by reintroducing some of the variation rather than taking the median case twice.)