This was the proof that I posted:
I am coming out of self-imposed exile from the P&R Forum to start this
thread. I don't intend to make posts in any other thread or to start any
other threads any time soon.
Title:
Theorem for a cold, snowy winter
Notation:
Where E is any event in a sample space, let P(E) represnt the
probability of E; let ~E represent the complementary event, that
E does not happen; and let P(~E) represent the probability of ~E.
Where E and F are events in a sample space and where F is possible,
let P(E|F) represent the conditional probabilty of E, given that F has
already happened.
THEOREM
Let A and B be events in a sample space. Let B be possible but not certain.
Then:
If P(A|B) > P(A), then P(A|~B) < P(A).
Statement of the theorem in words: If B happening makes A more probable,
then B not happening makes A less probable.
Proof of the theorem:
Let P(A) = a and P(B) = b. Note P(~B) = 1-b.
Let P(A and B) = x.
Recall that P(A|B) = x/b and we are given that this is greater than a.
Therefore, x > ab
Let d = x-ab. We know that d>0.
Note that P(A and ~B) = a-x = a-ab-d
Now note that P(A|~B) = P(A and ~B)/P(~B) = (a-ab-d)/(1-b)
= (a-ab)/(1-b) - d/(1-b) = a - d/(1-b) < a = P(A)
where the inequality is true because both d>0 and 1-b>0 (because B is not
certain).
quod erat demonstrandum
Why is this in the Politics & Relgion Forum?
Let A be "Anthropogenic global warming is a real problem."
Let B be "This winter is warmer than normal."
Anybody who can't see the point is innumerate.