The accuracy or, more correctly, the error of a measurement of discharge may be defined as the difference between the measured flow and the true value. The true value of the flow is unknown and can only be ascertained (within close limits) by weighing or by volumetric measurement. An estimate of the true value has therefore to be made by calculating the uncertainty in the measurement, this being defined as the range in which the true value is expected to lie expressed at the 95% confidence level (Fig. 13.1). Although the error in a result is therefore, by definition, unknown, the uncertainty may be estimated if the distribution of the measured values about the true mean is known.