It is assumed that the arrival of the number of calls X per hour follows Poisson distribution with parameter $\displaystyle \lambda$. A random sample $\displaystyle X_1 \; = \; x_1$ , $\displaystyle X_2 \; = \; x_2$ , ... $\displaystyle X_n \; = \; x_n$ is taken . Obtain the maximum likelihood estimate of the average arrival rate.