NOTE 1 The concept of 'measurement error' can be used both a) when there is a single reference quantity value to refer to, which occurs if a calibration is made by means of a measurement standard with a measured quantity value having a negligible measurement uncertainty or if a conventional quantity value is given, in which case the measurement error is known, and b) if a measurand is supposed to be represented by a unique true quantity value or a set of true quantity values of negligible range, in which case the measurement error is not known.

NOTE 2 Measurement error should not be confused with production error or mistake.

ANNOTATION (informative) [5 June 2014] Measurement error is sometimes understood as a quantity, which has a value, rather than as a value of a quantity.

ANNOTATION (for Note 1) (informative) [5 June 2014] The concept of 'measurement error' can be used either for a) or for b), but not for both a) and b) simultaneously.