There are 60 infections in village A per month and 48 infections in village B per month. Let A be no of infections in village A per month and B be no of infections in village B per month. Assume occurrence is independent and random. So Method 1 (Working method): A~Po (60) and B~Po (48) Using a suitable approximation, find the probability that in 1 month, the no of infections in B exceeds no of infections in A. Since λ>10, A~ N(60,60) and B~N(48,48) approximately B-A~(-12, 108) P(B>A)=P(B-A>0)=0.115 This I can understand but when I tried another method, it didnt work. Method 2: Through linear combination of poisson (???Cannot get it to work???) A-B ~ Po(12) Since λ>10, A-B~N(12,12) P(A<B)=P(A-B<0)=2.66x10-4 Why the difference? Why does the second method not work?