is the natural sufficient statistic.

Sometimes, the variance of an estimator is compared to the variance of some other estimator (rather than the Cramér-Rao bound): For two unbiased estimators and of the same parameter , the estimator would be called more efficient than if , irrespective of the value of . One can also say that the most efficient estimators are the ones with the least variability of outcomes. The notion of (relative) efficiency can be extended to biased estimators by saying that an estimator is more efficient than