4. A random sample $X_1, X_2, \ldots, X_n$ of size n is taken from a Poisson distribution with a mean of $\lambda, 0 < \lambda < \infty$. Show that $\bar{X}$ is the minimum-variance unbiased estimator for $\lambda$. (15%)
4. A random sample $X_1, X_2, \ldots, X_n$ of size n is taken from a Poisson distribution with a mean of $\lambda, 0 < \lambda < \infty$. Show that $\bar{X}$ is the minimum-variance unbiased estimator for $\lambda$. (15%)