If a is true in 2,000 samples, approximately how many samples will have both a and b true?
a is true in 2,000 samples, and b is true in each of these with probability P(a|b)=0.0015. Therefore, the expected number of samples with both a and b true is 0.0015*2000 = 3. This is a very small number, and is unlikely to yield accurate estimates if b happens to be part of our evidence.