Estimator

In statistics, an estimator is a rule for calculating an estimate of a given amount based on observed data. The rule (the estimator), the amount that is being measured (the estimand) and its result (the estimate) are different.^[1]

An estimator of a parameter $θ$ is often written as $\hat{θ}$ .^[2] If the expected value of the estimator is equal to the parameter, then the estimator is called unbiased. Otherwise, it is called biased.^[3]