by emailking » Mon 12 Jan 2009, 20:04:49
$this->bbcode_second_pass_quote('jbeckton', '
')
Yes, the probability of collapse increases after the building is set on fire, therefore:
P(B) does not equal P(B|A)
He's trying to argue that the probability of the concurrence of two events is smaller than the probability of either event. This is simply true. (unless one of the probabilities is unity in which case equality may hold) Showing this does not depend on assuming P(B) = P(B|A)
It's not hard to see this. In general
P(A and B) = P(A)*P(B|A) <= P(A) since P(B|A) <= 1
But P(A and B) = P(B and A) = P(B)*P(A|B) <= P(B) since P(A|B) <= 1
$this->bbcode_second_pass_quote('jbeckton', '
')This is an assumption you have made (you know what happens when you make assumptions). I have done probability (PRA) studies for nuclear power plants that have probabilities 10^(-6) and smaller which is certainly <<1, are you saying we should ignore them? The NRC says they are significant, I agree.
He's not saying to ignore them (that would entail setting them to 0). If they are less than 1, that constitutes "sufficiently small" for this analysis.
')Have you made any attempt to quantify P(B) and P(B|A) before assuming the difference is negligible? I bet you haven’t.