Bessel is responsible for the correction to the formula for the sample variance estimator named in his honour. This is the use of the factor "n" − 1 in the denominator of the formula, rather than just "n". This occurs when the "sample mean" rather than the "population mean" is used to centre the data and since the sample mean is a linear combination of the data the residual to the sample mean overcounts the number of degrees of freedom by the number of constraint equations — in this case one. "(Also see Bessel's correction)".