Hi

Thanks for getting back to me.

I got this online for the pmf of a poisson.

http://upload.wikimedia.org/math/5/e...0851b75ad7.png
I still don't see how you get

1- Pr(0) + Pr(1) + Pr(2)= 1-(e^-3 +3e^-3 + 9/2 e^-3)

could you provide me with the formula into which I sub the values please. I see that on the left it's each value for the probability up to and including 3 and then subtract from 1 to get probability of greater than or equal to 4, but the 9/2 on the right hand side of the equation? No idea where that came from.