The error of measurement is the difference between a measured value of a quantity and its true value. The term measurement uncertainty is often used as a synonym for measurement error. In metrology, the analysis of errors includes the study of uncertainties in the measurements, since no measure as far as it is carried out with care is entirely free from uncertainties.
The term error does not necessarily imply an incorrect measurement procedure by the operator, but also an uncertainty provided by the instrumentation, namely that the value presented by the measuring instrument provides a value of the measured quantity with a certain approximation. Measurement errors are caused by:
- human factors (inaccuracies in the design of the measurement chain, distractions or poor operator accuracy);
- technological factors (static and dynamic constructive and metrological qualities of the instruments);
- environmental factors (external influence quantities present in the environment in which the measurement is made).
In statistics, an error is not a “mistake”. Variability is an inherent part of the results of measurements and of the measurement process. The measurement operation is always invasive, in fact, it introduces a perturbation in the system that we want to investigate; therefore the variables involved are always altered when the measurement is performed.
Classification of measurement errors
The measurement error can depend on both the instrument and the observer. There are two main types of errors:
- random errors (or accidental, which may vary from observation to another);
- systematic errors (which always occurs, with the same value, when we use the instrument in the same way and in the same case).