Uncertainty: Helpful Resources

On this page you'll learn of some official advice about "doing uncertainty".

It may surprise you to learn that there isn't actually a formal standard for how general-purpose measuring instruments are specified, or how those specifications are arrived at. Furthermore, when "calibration to manufacturer's specifications" is requested, a common assumption is that all cal labs use a standard method to qualify their calibration capability -- in other words, whether the test method/equipment is adequate for the intended purpose. Except for ISO/IEC17025 accredited calibration, this is not the case. Adequacy has often been judged by a relatively simple TAR (test accuracy ratio) approach, considering only the principal accuracy specification of the test gear rather than the total uncertainty in the calculation.

ISO GUM

Technical Advisory Group 4 / Work Group 3
Guide to the Expression of Uncertainty in Measurement
ISBN 92-67-10188-9

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. Conference organizers need only include an "Uncertainty" topic in the agenda to instigate animated audience participation !

• ISO

Practical Guides for Technicians

Before, and after, release of the GUM several national guides have been produced which aim to provide practical advice for those equipment users who are more likely to be at technician, rather than scientist, level.

EA-4/02 Expression of the Uncertainty of Measurement in Calibration

Released in December 1999, this is a comprehensive (79 pages)  and authoritative publication (from Europe's official accreditation organization). Other than price (FREE), what makes this especially valuable is the number of examples that are used to illustrate the uncertainty budgeting process.

• EA-4/02

UKAS M3003

The first, widely acknowledged practical guide was undoubtedly the old British Calibration Service's document B3003 that addressed electrical calibrations and which subsequently became NAMAS NIS3003 and later renamed M3003. The latest 82-page version (edition 2, January 2007) is consistent with the GUM and EAL G3 (which replaced WECC document 19) also includes examples from the non-electrical fields. The good news is that UKAS publish it in PDF form and it's available to anyone to download for FREE !

• UKAS M3003

NIST Guide

The US National Institute of Standards and Technology published their own document to aid understanding of the ISO's, rather daunting, GUM. In outlining their policy and defining the practice for NIST's own staff, NIST Technical Note 1297 simplifies The Guide to 15 pages. Better still, their website provides an even more gentle introduction to the subject of uncertainty from which you can also access TN1297 when more detail is required.

• NIST's Introduction to Uncertainty
• Technical Note 1297

SAC-SINGLAS Technical Guide 1

First produced in 1995, it includes a large collection of examples from various disciplines and, for that alone, is a valuable resource. After going out-of-print, it was unavailable for several years but the second edition was published in March 2001. It may be obtained, in hardcopy form only, at a cost of about $41 (Singapore) by contacting:

Singapore Accreditation Council, 2 Bukit Merah Central Singapore 159835.
Tel : +65 6279 1856

A shortened version is available for FREE on their website.

• Guidance Note EL 001- May 2002

Uncertainty Software

Several programs are available in the Download section (under the Explore banner above right) to help you with the task.