This is an error that remains constant in absolute value and sign when several measurements of the same fixed quantity are made under the same conditions.

The causes of systematic errors may or may not be known. If such an error can be determined by calculation or experiment, an appropriate correction is applied to the measurement result. If a systematic error cannot be determined but if it can be assumed to be sufficiently small by comparison with the imprecision of measurement, it is assessed approximately and taken into account in the calculation of the error.

Example of a systematic error: measurement of a mass using a supposed 1kg test weight, where the true mass is 1.005 kg.