In: Statistics and Probability
During rush hour, from 8 am to 9 am, traffic accidents occur according to a Poisson process with a rate of 5 accidents per hour. Between 9 am and 11 am, they occur as an independent Poisson process with a rate of 3 accidents per hour. What is the PMF of the total number of accidents between 8 am and 11 am?
Let X be the random variable that denotes the number of traffic accidents occurring between 8 am and 9 am.
X Poisson (1 = 5)
Let Y be the random variable that denotes the number of traffic accidents occurring between 9 am and 11 am.
Y Poisson (2 = 3)
X and Y are independent poisson random variables.
By the addition of independent poisson processes, let Z = X + Y
So, Z Poisson ( = 1 + 2)
Z Poisson ( = 5 + 3 = 8)
The pmf of the total number of accidents between 8 am and 11 am is
P(Z = z) = (e-8 * 8z) / z! ; z = 0, 1, 2, ...........
= 0 ; otherwise