A prescription for obtaining an estimate from a random sample of data. An estimator is a procedure, not a result, and therefore is a random variable and has a variance. For instance, an estimator of the mean weight in adult men may consist of the prescription “Add up the weights of 100 men and divide by 100.” The actual outcome (the estimate) will vary from sample to sample, but one answer will not be a random variable.
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Examples: glitazone, GI cocktail, etc.