 If "n" is unknown, then the maximum-likelihood estimator of "n" is "X", even though the expectation of "X" given "n" is only ("n" + 1)/2; we can be certain only that "n" is at least "X" and is probably more.In this case, the natural unbiased estimator is 2"X" − 1.The theory of median-unbiased estimators was revived by George W. Brown in 1947