Bias

An effect which deprives a statistical result of representativeness by systematically distorting it, as distinct from a random error which may distort on any one occasion but balances out on the average. The bias of an estimator is the difference between its mathematical expectation and the true value it estimates. In the case it is zero, the estimator is said to be unbiased. OECD Source: The International Statistical Institute, "The Oxford Dictionary of Statistical Terms", edited by Yadolah Dodge, Oxford University Press, 2003.