1. Statistical efficiency compares two unbiased estimators by calculating their relative variance. The estimator with the lower variance is more efficient. 2. The Cramer-Rao inequality provides a lower bound (CRLB) for the variance of an unbiased estimator. It states that the variance of an estimator must be greater than or equal to the inverse of the expected information. 3. An estimator that achieves the Cramer-Rao lower bound is considered statistically efficient, as its variance reaches the theoretical minimum. In the example given, the maximum likelihood estimator of the mean of a normal distribution is shown to be efficient.