a quantity calculated from sample data whose value is representative of the true quantity in the larger population. In other words, when data from samples are used to make inferences about unknown quantities (parameters) in populations, an unbiased estimator is one that over repeated sampling has an average equal to the true value of the parameter in the population. For example, an unbiased estimator of variance provides an accurate index of the variability of measurements for a given phenomenon in a given population of interest based on calculations made from the sample variance. Compare biased estimator.