Poisson mean scaling

When we’re trying to solve problems involving the poisson distribution, we might have a question which involves comparing two things that don’t have the mean set to the same scale.

For example, we might know the mean number of cars that pass through a junction in 1 hour, but we want to know the probability that a certain number of cars pass through in 30 minutes.

To solve these problems, we can just scale the mean up/down to the new, correct, time period.

For our example, if we know that the mean number of cars that pass through a junction in 1 hour is 10, then the mean number of cars that pass through in 30 minutes is just half of that, so an average of 5 cars per 30 minutes.

We can’t scale our value of x up/down. So if we wanted to find the probability of 3 cars passing in 30 minutes, we can’t scale 3/30min up to 6/1hr. We need to keep x as 3 and just scale the mean down to 5.