Service Time DistributionThe exponential distribution describes the probability that the service time of the customer at a particular facility will be no more than T time periods. The probability can be calculated by using the formula The mean of the service time distribution is 1>m, and the variance is (1>m)2. As T increases, the probability that the customer’s service time will be less than T approaches 1.0. For simplicity, let us look at a single-channel, single-phase arrangement.