Dear Lazyweb

[jpmassar]

If a murder occurs in Oakland, CA approximately once every three days, and the interarrival time between murders is assumed to be Poisson distributed, then what is the probability that in some 18 hour interval over the year-to-date (or over the whole year) that there will have been at least 5 murders?

Is there a standard formula to figure this out, given an interrarival time, a long interval, a short interval and a threshold?