Poisson Distribution Calculator

The Poisson distribution gives the chance of seeing a number of events in a fixed interval when they arrive independently at a steady average rate. Enter lambda, the mean number of events in that interval, and a count, and it returns the probability of exactly that count together with the totals above and below it. A second action builds lambda from a rate and an interval length, and a third adds up a whole range of counts. Every probability is computed in logarithms, so a count in the hundreds does not overflow. The answer assumes the rate never changes and that events do not arrive in clusters, which is where real data usually parts company with the model.