A measurement error is the uncertainty with which a displayed measured value is afflicted and deviates from the actual value of the measurand. A distinction is made between absolute and relative measurement errors.
An absolute measurement error results from the difference between the measured value and the actual value. The relative measurement error is related to a quantity. It is dimensionless and is calculated from the quotient of the measured value to the reference value.
The reference quantities can be the correct value, the displayed value or the full scale value. Relative measurement errors are expressed in percent, in parts per million( ppm) or with a power of ten (`10^-3`).
Measurement errors have many causes, which lie in the measurement procedure, the measured object, the accuracy of the measuring instruments, the reading accuracy or external influences. Accordingly, measurement errors can be systematic or random. Systematic measurement errors are those in which all measurements have the same systematic errors. These can be errors in the measurement setup such as an incorrect ground connection or a ground loop, or the internal resistance of the measuring instrument falsifies the measurement result. The measurement error remains with all measurements. In contrast, random measurement errors tend to occur sporadically and are only noticed by the person performing the measurement when a different measurement result is displayed for the same measurement.