NOTE 1 Metrological compatibility of measurement results replaces the traditional concept of 'staying within the error', as it represents the criterion for deciding whether two measurement results refer to the same measurand or not. If in a set of measurements of a measurand, thought to be constant, a measurement result is not compatible with the others, either the measurement was not correct (e.g. its measurement uncertainty was assessed as being too small) or the measured quantity changed between measurements.

NOTE 2 Correlation between the measurements influences metrological compatibility of measurement results. If the measurements are completely uncorrelated, the standard measurement uncertainty of their difference is equal to the root mean square sum of their standard measurement uncertainties, while it is lower for positive covariance or higher for negative covariance.

ANNOTATION (informative) [2 December 2014] Here "measured quantity values" can be replaced by "measured values" and "standard measurement uncertainty" can be replaced by "standard uncertainty" without ambiguity: "property of a set of measurement results for a specified measurand, such that the absolute value of the difference of any pair of measured values from two different measurement results is smaller than some chosen multiple of the standard uncertainty of that difference".