Glossary

The IAG guide to the use of metrological terminology

Introduction

All areas of science – indeed all fields of human endeavour, be it the law, architecture or music – develop their own terminologies. Metrology and geochemistry are no exceptions.
Ideally, such terminologies aid progress by facilitating clear, unambiguous written and verbal communication between practitioners. Unfortunately, terminologies are not usually static. They evolve over time as advancements are made in their field. New terms appear and old definitions are superseded. Moreover, the same term may be used in two completely different areas of knowledge and have two completely different meanings in each. The classic example is standard, which is used in both analytical science, often referring to some reference value, and also in many fields of technical regulation where it refers to a document listing many requirements that must be met.

Scientists whose research involves measurements have evolved specific terminology to describe and communicate the work they undertake. The basis for modern metrology is the SI system (International System of Units). However, accompanying this measurement system, there is a need to ensure international agreement in the terms used to describe various stages of the measurement processes.

The responsibility for developing a standard international system of terminology rests the Joint Committee for Guides in Metrology (JCGM), established in 1997 and chaired by the Director of the Bureau International des Poids et Mesures (BIPM) in Sèvres, France.

The main output of the JCGM with respect to measurement terminology has been the international vocabulary of metrology (VIM). This has been through various revisions, the latest of which is VIM 3 (International vocabulary of metrology – basic and general concepts and associated terms, JCGM 200:2012, 91pp, see http://www.bipm.org/en/publications/guides/vim.html).

VIM3 provides a rigorous definition of almost all terms required for use in the measurement sciences, sometimes with guidance for use.

Experience has shown that difficulties have arisen in the use of the terminology listed in VIM3 . As a result, the majority of draft papers submitted to analytical journals for publication contain errors in metrological terminology. These errors arise from a number of sources:

  • The VIM definitions are metrologically rigorous, but some are written in a way that is difficult to interpret by a practical analyst
  • This problem is compounded by the fact that the definitions are designed to be fit for purpose for all the chemical, physical and medical sciences
  • Some concepts are based on new terms that are not in common usage, e.g. ‘traceability’
  • Other concepts are based on words that are in common use, but are given a very specific metrological meaning, e.g. ‘accuracy’
  • Possibly out of frustration, some sectors of the measurement sciences have developed substitute terminology in circumstances where a perfectly good VIM 3 definition exists, e.g. ‘internal precision’
  • Science journal editors have not always been rigorous in insisting on the correct use of VIM3 terminology. Advice has been sought from the journal Geostandards and Geoanalytical Research concerning some of the most misused terms.

The purpose of this glossary is, therefore, to explain and provide advice on the correct use of metrological terminology in a simple and accessible way. The overall aim is to allow readers to improve the quality of their research manuscripts in the way that measurements are communicated.


 

Index to terms

Concept

Metrological terms considered

Metrological terms covered

Say no to ppm

The ambiguities associated with the use of ‘ppm’

 

Sigma is out, standard deviation is the way to go!

What symbols are used to represent the properties of population- and sample-distributions

 

Standard ≠ reference material

Avoiding the use of the term standards when referring to (certified) reference materials or calibrators

Reference material, certified reference material, standard reference material, calibrator, calibration, validation, measurement standard (étalon), verification

Please, no more errors from your laboratory

Explaining the difference between uncertainty and error

Measurement uncertainty, measurement error, systematic measurement error, measurement bias, random measurement error, confidence level

Precision

Distinguishing between repeatability, intermediate precision and reproducibility and discouraging the use of ‘internal precision’

Measurement precision, repeatability condition of measurement, intermediate precision of measurement, reproducibility condition of measurement, repeatability, intermediate precision, reproducibility

Small bias yes, high accuracy no

Explaining the difference between accuracy, bias and trueness

Measurement accuracy, measurement trueness, measurement bias

REFERENCES

VIM3
International vocabulary of metrology: Basic and general concepts (VIM). Third edition. JCGM 200:2012, 91pp (http://www.bipm.org/en/publications/guides/vim.html)

GUM
Evaluation of measurement data - Guide to the expression of uncertainty in measurement. JCGM 100:2008, 120pp (http://www.bipm.org/en/publications/guides/gum.html)

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