To illustrate this property, sup that the intervals of time. in minutes. between arrivals of successive customers at a certain service facility are i.id. random variables Suppose also that each interval has an exponential distribution with parameter B. and that a set of observed intervals x Xn form a random sample from this distribution 1... It follows from Exercise 7 of Section 6.5 that the M.I. F. of B will be 5 m 11 X,.. Also since the mean t of the exponential distribution is l.F. it follows from the imvariance property of MLE's that Xn. In other words. the MLE. of the mean is the average of the observations in the sample Consider now the following three sampling plans: