Poisson distribution

The Poisson distribution counts how many times something happens in a fixed window of time or space, assuming events arrive independently at a steady average rate $\lambda$ (lambda). If a help desk averages 3 calls per hour, the probability of exactly 2 calls in an hour is $\frac{3^2 e^{-3}}{2!} = \frac{9(0.0498)}{2} \approx 0.224$. The general formula is $P(X = k) = \frac{\lambda^k e^{-\lambda}}{k!}$, and both the mean and the variance equal $\lambda$. The Poisson distribution is not part of AP Statistics at all, though it is standard in college probability and statistics courses.