Abstract: A method of determining the uncertainty value of an electrical measuring instrument, such as a digital voltmeter. The method comprises the steps of storing digital data values representative of the accuracy specifications of the instrument as a function of the measured reading and as a function of the full scale reading and supplying the data values to a data processing apparatus. A digital value representative of a measured reading is also supplied to the data processing apparatus which is then arranged to calculate the uncertainty value of the measured reading and to display the calculated value. Apparatus for determining such an uncertainty value is also disclosed.