Elevator passenger arrivals. A study of the arrival process of people using elevators at a multilevel office building was conducted and the results reported in Building Services Engineering Research and Technology (Oct. 2012). Suppose that at one particular time of day, elevator passengers arrive in batches of size 1 or 2 (i.e., either 1 or 2 people arriving at the same time to use the elevator). The researchers assumed that the number of batches, n, arriving over a specific time period follows a Poisson process with mean l = 1.1. Now let xn represent the number of passengers

(either 1 or 2) in batch n and assume the batch size has probabilities p = P1xn = 12 = .4 and q = P1xn = 22 = .6.

Then the total number of passengers arriving over a specific time period is y = Σi = 1 n xi. The researchers showed that if x1, x2, …, xn are independent and identically distributed random variables and also independent of n, then y follows a compound Poisson distribution.

a. Find P1y = 02, i.e., the probability of no arrivals during the time period. [Hint: y = 0 only when n = 0.]

b. Find P1y = 12, i.e., the probability of only 1 arrival during the time period. [Hint: y = 1 only when n = 1 and x1 = 1.]