Definition An estimator is said to be unbiased if and only if where the expected value is calculated with respect to the probability distribution of the sample. Bias can also be measured with respect to the median rather than the mean in which case one distinguishes median-unbiased from the usual mean-unbiasedness property. In statistics bias is an objective property of an estimator.
Where X is the sample mean.
Of an estimator having an expected value equal to the parameter being estimated. For example the mean of a sample is an unbiased estimate of the mean of the population from which the sample was drawn. Practice determining if a statistic is an unbiased estimator of some population parameter. Probability and statistics symbols table and definitions - expectation variance standard deviation distribution probability function conditional probability covariance correlation.