It may surprise you to learn that there isn't a formal standard for how general-purpose measuring instruments are specified, or how those specifications are determined. Furthermore, when “calibration to manufacturer's specifications” is requested, a common assumption is that all calibration laboratories use a common method to qualify their calibration capability. Except for ISO/IEC17025 accredited calibration, this is not the case.

“Measurement adequacy” has often been judged by a relatively simple TAR (test accuracy ratio) approach, considering only the principal accuracy specification of the test equipment rather than all of the contributors to the total uncertainty.

The “Guide”, or GUM as it has become known, is a 100+ page document which recommends a methodology for qualifying test accuracy (or more correctly, the potential test inaccuracy) and was published in 1993. Although comprehensive in its treatment of the subject, it doesn't enjoy universal acclaim. Statisticians say it is critically flawed. Practicing engineers view it as too complicated and lengthy. It is, nevertheless, the first methodology to achieve formal, international consensus after decades of debate.

ISO/IEC Guide 98-3:2008 Guide to the Expression of Uncertainty in Measurement

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OIML G 1-100:2008 – The Free Version of the GUM

A free version of the GUM is available from the International Organization of Legal Metrology. Download it by visiting and search explicitly for “Evaluation of measurement data ‑ Guide to the expression of uncertainty in measurement”.

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