um
in formulas.doc
page 2
3. P (A or B) = P (A) + P (B) – P (A and B)
4. P (A and B) = P (A) + P (B)
with regards to 3:
if mutually exclusive, P (A or B) = P (A) + P(B)
if not mutually exclusive, P (A OR B) = P ( A ) + P ( B ) - P (A and B)
though thats not a major error, I guess it would be logical and your formula would work. Just a technicality
anyway
major problem is with 4. its v confusing, and from what I understand, its wrong.
P (A AND B) = P(A) + P (B) <-------- ??
people might confused that with the product rule. and, in any case, shouldn't it be:
P (A AND B) = P (A + B)
or
P (A and B) = P (A U B)
if you think about it, look at 3 and 4. you're saying P (A or B) = 0
since P(A) + P(B) - ( P (A) + P (B))
you should delete 4 all together.
