The basis of ISO GUM is the Recommendation INC-1, 1980 . The uncertainty in the result of a measurement generally consists of several components, which may be grouped into two categories according to the way in which their numerical values are estimated.
(A) Those which are evaluated by statistical method (In this case, the standard deviation is calculated by taking square root of the mean of sum of squares of deviations from the mean). This is the case of repeated observations.
(B) Those which are evaluated by other means.
There is not always a simple correspondence between the classification into categories A or B and the previously used classification into “random” and “systematic” uncertainties. The term “systematic uncertainty” can be misleading and should be avoided.
Any detailed report of the uncertainty should consist of a complete list of the components, specifying for each, the method used to obtain its numerical value.
The components in category A are characterized by the estimated variances s2p (or the estimated standard deviations sp) and the number of degrees of freedom. When appropriate, the estimated covariance should be given.
The components in Type B should be characterized by quantities u2 j , which may be considered as approximations to the corresponding variances, the existence of which is assumed. The quantities u2 j may be treated like variances and the quantities uj like standard deviations. Where appropriate, the co-variances should be treated in a similar way.
The combined uncertainty should be characterized by the numerical value obtained by applying the usual method for the combinations of variances. The combined uncertainty and its components should be expressed in the form of “standard deviations”.
If, for particular applications, it is necessary to multiply the combined uncertainty by a factor to obtain an overall uncertainty, then the multiplying factor used must always be stated.
As a follow-up action, the international Organization for Standardization (ISO), the International Electro-technical Commission (ICE), the International Federation of Clinical Chemistry and Laboratory Medicine, the International Union of Applied and Pure Chemistry (IUPAC), the International Union of Pure and Applied Physics (IUPAP), the International Organization of Legal Metrology (OIML) and the International Bureau of Weights and Measures (BIPM) joined hands to form a Joint Committee for Guides in Metrology (JCGM) to produce a document. The document “Guide to the Expression of Uncertainty in Measurement (GUM)” wasfirst published in 1993 and reprinted in 1995 with minor corrections .
