Efficiency:

For an unbiased estimator, efficiency indicates how much its precision is lower than the theoretical limit of precision provided by the Cramer-Rao inequality. A measure of efficiency is the ratio of the theoretically minimal variance to the actual variance of the estimator. This measure is usually not higher than 1. An estimator with efficiency 1.0 is said to be an "efficient estimator".

The efficiency of a given estimator depends on the population. For example, for a normally distributed population, the sample mean is an efficient estimator of the population mean. But the sample mean is usually not an efficient estimator of the mean of a non-normal population.

The efficiency of an estimator should not be confused with the relative efficiency (of statistical tests).