Probable value of the measured (quantity x) is defined as the arithmetic mean value of all performed equally accurate measurements n some one of the same numerical values of physical quantities. Thus, the most probable value of the measured physical quantity:

where — the sum of the values obtained in the measurement.

Accuracy and reliability was assessed by measuring the relative error by comparing the absolute value of the error of measurement of the measured value, given confidence probability.

Absolute error Δx representing generally the difference between the desired values and the results of its measurement really can not exhaustively characterize the accuracy of measurements, since they depend on the numerical value of the measured value. Since the true value of the measurand generally remains unknown, for practical calculations take into account its probable value:

At the same time take into account that the possibility of receiving parcels of plus and minus sign and the same amount of random errors, therefore close to zero.

Indicators measuring accuracy are rms values, and average probable error.

Rms measurement error σ individual:

where the sum of squared deviations of the results of individual measurements from their average value.

Note that the relative value of the mean square error expressed as a percentage, called the coefficient of variation: