X = # of cars that pass in one hour E(X) = λ = n * p λ cars/1hour = 60min/hour * (λ/60) cars/min In this old video (5:09) on poisson process Sal asks: "What if more than one car passes in a minute?" "We call it a success if one car passes in one minute, but even if 5 cars pass, it counts as 1 car." I don't understand the statement in bold. Why do 5 cars count as 1 car? I do understand that λ/60 = the probability that a car passes in a minute (the probability of success in a minute) and that we make it more accurate if we would again decrease the interval to 3600 seconds instead of 60 minutes. As I wrote this, I think I got it by typing "interval". Does it count the event of '5 cars in one minute' as '1 car in one minute' because the one minute interval limits the interpretation of success to "car/minute = success"? (As in: as long as 0<cars = success, and then discards the actual number of cars which passed) Sorry for the likely unclear explanation, but I was/am terribly confused by this probably obvious fact.