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DISCUSSION FORUM Papers published in this section do not necessarily reflect the opinion of the Editors, the Editorial Board and the Publisher. Apart from exceptional circumstances, they are not submitted to the usual referee procedure and go essentially unaltered.
Accred Qual Assur (2003) 8:475–476 DOI 10.1007/s00769-003-0599-7
Gary Price
Traceability to units Published online: 9 April 2003 © Springer-Verlag 2003
Abstract On the grounds that clear and direct communication is required of us today, it is proposed that traceability be regarded as the ability to demonstrate that measurements are what they are purported to be and that traceability is thus to measurement units rather than reference values per se. It is suggested that such an approach may give greater flexibility in the establishment, maintenance and propagation of traceability, and that accreditation practices are becoming central to the practical establishment of traceability for chemical and biological measurement. Keywords Traceability · Units · Reference value · Metrology · Accreditation · Scale
Communicative accuracy The wider society of measurement users is anybody and everybody who makes, or is affected by, a decision dependent upon a measurement. It is literally everybody. The decisions large and small, in their teeming multitude, are made by men and women with a vast variety of knowledge and experience, but seldom directly relevant to the technical issues of the measurement. Through a fog of self- and competing interest, and particular circumstance and need, they must summon sufficient understanding to make informed decisions. They are the audience whenever we write down a measurement result. The dilemmas of communicating to the public are not of course unique to chemical and biological measurement. Warren Weaver, one of the founders of the modern theory of communication, proposed to the scientist struggling with communication problems the concept of communicative accuracy [1]. It rests on the fact that the effective accuracy of a communication depends primarily on the interpretation given to it by the audi-
ence. Weaver suggested two conditions for communicative accuracy: First, taking into account what the audience does and does not know, it must take the audience closer to a correct understanding..… Second, its inaccuracies (as judged at a more sophisticated level) must not mislead.... Both of these criteria must be applied from the point of view of the audience, not from the more informed and properly more critical view of an expert. Viewed through these criteria, there is considerable room for improvement in our communication practices in chemical and biological measurement. Anyone who wishes to dispute this should contemplate how they would explain to a jury in the face of a persistent and skillful opposing advocate the concepts of the mole and amount of substance as defined in the SI.
Traceable to what? Traceability is a concept intimately involved in the communication of measurement results and their meaning, but what is the wider society of users of measurement results to make of it? On the one hand they are told of its central importance and the need to have an infrastructure to enable and support it, and on the other hand, confusion, bewilderment and ambiguity as to what it is and most especially, what it is to. They are told that traceability is a “property of the result of a measurement or the value of a standard whereby it can be related to stated references, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties” [2] (my emphasis). Yet they find talk of traceability to: pieces of paper or certificates, laboratories or institutions, methods or instruments, pure substances, certified reference materials, reference values, “agreed” reference standards, and the delightfully bureaucratic “appropriate” reference standards. We all know about the multitude of ambiguities attached to the English word “standard”, but this is surely ridiculous. To start with, most of the end points listed are not even the sorts of things that measurements can be traceable to, at least on the face of it. To say that laboratories or substances are properties of measurements per se and to which they may be compared is a simple confusion of logical categories, like saying that a velocity can be compassionate. Indeed,
strictly and literally speaking, the only member of the list that lies unambiguously in the universe of possibilities for traceability end points is “reference values” [3]. Not unreasonably it will be objected that this is an overly literal interpretation: that when we say a measurement is traceable to, for example: a laboratory; or a pure substance; or a certified reference material, we are speaking shorthand code for something respectively like: “reference value determined or maintained” by a laboratory; “reference value created by weighing out a sample” of a pure substance; or “reference value stated in an attached certificate” of a certified reference material. It is not an unreasonable approach in specific cases of specific measurement situations when technical peers talk among themselves about specific problems. But as a general explanation to the wider society of measurement users it is both beside the point and misleading, for at least two reasons. The first reason is that the wider society quite rightly demands transparency and trust in the communication of information essential to such fundamental purposes as production, trade and commerce, health care, environmental policies, and legal judgement. Speaking in codes to your audience is not one of the more notable ways to achieve transparency and trust. The wider society is entitled to ask what is really going on with this linguistic jiggery pokery, and whose interests does it serve? More plain speaking and less – not more – obfuscation are what is required of us. The second reason is that it is incomplete and leaves the traceability chain dangling like a metrological rope trick. On being informed that traceability is to reference values which may be in a variety of material forms, an intelligent but technically unsophisticated measurement user is entitled to say “Yes, I think I understand that, but what’s the point? What is all this complex chain of comparison actually for? What is its purpose? And why is traceability such a good way of doing it?” You will generally have no more than 30 seconds to answer before your audience turns their mind to more pressing matters, convinced only of the utter inconsequentiality of the subject.
A small proposal Allow a modest proposal: “Traceability is the ability to demonstrate that measurements are what they are purported to be”. Because measurements are always expressed and communicated in the form of numerical values (with associated uncertainties or equivalent intervals between numerical values, at stated levels of confidence) multiplied by measurement units, it then follows by ineluctable logic that the end point of any traceability chain is simply the units
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in which the measurement is expressed. What then, is the role of reference values? There is an older form of words that one still sometimes sees that infers that traceability is to “reliable realisations of the units” in which a measurement is expressed. The proposal here given is a close cousin to that view and is related also to Belanger’s approach that traceability is the means to ensure measurements of accuracy sufficient for the purpose at hand [4], an approach that Nicholas and White amended (with close consideration of the evolving role of accreditation) to the view that traceability is the ability to demonstrate the accuracy of a measurement in terms of its expressed units [5]. However those previous views adequately considered neither the complexities of practical chemical and biological measurement nor their highly instrumental nature, nor the variety of measurement units and scales that may be encountered. A more general formulation is that measurement scales are the means by which numbers are assigned to quantities that we may desire to measure and that scales are on the one (theoretical) hand, defined by measurement units; and on the other (practical) hand, realised in material ways by applying reference values to measurement procedures or instruments (sometimes termed “calibration”). Thus the suggestion being made here is that the construction of a measurement scale is the mediating step between the reference values that may be available to an analyst and the units with which the relevant measurement results are expressed. One reason to commend the approach suggested is that it gives a proper emphasis to the integrity of the whole measurement, not just the series of comparisons from measurand or analyte to reference values. In actual practice, the uncertainties with which reference values effectively realise units are on occasion omitted from consideration. This is sometimes justified with the claim that, if all is in order, the uncertainty of the reference value is negligible in comparison to that of all operations and influence factors from reference value to measurand. Far from being in order, I wish to suggest that the requirement that reference values have negligible relative uncertainties is a potential barrier to the advancement of practical metrology in chemistry.
Where do reference values come from? There is one thing that we can definitely say is not a source of reference values and that is interlaboratory comparison. Interlaboratory comparison, proficiency testing and the like are very useful tools, depending on the protocols and intended purposes. They can be used to detect and diagnose problems, evaluate competencies and pro-
ficiency, and even possibly propagate traceability, but they can never conjure traceability into existence. If traceability is not independently established, interlaboratory comparison is just as efficient and effective in propagating systematic error. One answer is that reference values ultimately derive from national measurement institutes, high level laboratories and facilities with the technical capability of making highly accurate primary measurements on carefully prepared materials and these are disseminated down the measurement chain as certified reference materials of progressively larger uncertainties. The Consultative Committee for Amount of Substance (CCQM) defined a primary method of measurement as “having the highest metrological qualities, whose operation can completely be described and understood, for which a complete uncertainty can be written down in terms of SI units” [6]. The central idea was that such a method was in a sense an absolute or defining measurement that could stand alone without reference or comparison to other standards of the same quantity and produce independent reference values. Its essence was the conceptual transparency implied by the terms “completely described and understood” so that a relatively simple equation could describe the measurement and all influence factors could be accounted for and their uncertainty evaluated. It is a very useful idea with large unrealised potentials, but nobody has yet given any cogent explanation of the phrase “highest metrological qualities”. Could this phrase possibly be a bit of special pleading in favour of oligarchic (if not monopolistic) supply of reference values? There is certainly no compelling reason in principle to say that a competently applied primary method but of larger than state-of-the-art uncertainty does not still produce a perfectly good reference value for many purposes. Historically, analytical chemistry more often than not relied on “do-it-yourself reference values”, by for example weighing out samples of pure materials, preparing “standard solutions” or creating experimental set-ups where known amounts of a species of interest are generated and the like. These may not have been of the “highest metrological quality” but they were certainly fit for their purpose then. Of course, that was decades ago, when analytical life was very, very much simpler and many of our now commonplace instrumental methods did not exist. But many modern instrumental methods are also potentially highly precise-far more precise than many of their practical field uses may require. Might higher uncertainties in the reference values applied to them still result in overall uncertainties at least adequate to the purpose? What is contemplated here is a flattening of the practical metrological pyramid, a shortening of the distance between units and their expression, a reduction of the
number of comparisons in the traceability chain and a degree of metrological selfsufficiency where laboratories or networks of laboratories could, if it makes sense in their circumstances, construct as directly as possible their own intrinsic realisations of units or reference values, appropriate to their purposes but traceable to units in common with (and hopefully understood by) the rest of the world.
Conclusion Reference values may be propagated in many ways. Traditional certified reference materials are just one of them. One could for instance imagine electrochemical amount generators, or reference laboratories able to perform matrix-independent reference measurements on samples submitted by field laboratories, the samples then being returned to the field laboratory with a reference value attached. There are many possible paths to metrological virtue. The requirements of communicative accuracy suggest they converge on measurement units. What is essential to all of them nowadays is summarised in the first part of the proposed definition of traceability: “..is the ability to demonstrate..” Accreditation practices, the trust and transparency mechanisms of modern practical measurement systems are I would suggest now central to the establishment of practical traceability for chemical and biological measurement.
References 1. Weaver W (1967) Science and imagination: selected papers of Warren Weaver. Basic Books, New York London, pp 183–184 2. ISO (1993) International vocabulary of basic and general terms in metrology. ISO, Geneva 3. De Bievre P (2002) Accred Qual Assur 7:515 4. Belanger BC (1980) Bulletin OIML 78:21–25 5. Nicholas JV, White DR (1994) Traceable Temperatures: an introduction to temperature measurement and calibration, 1st edn. Wiley, New York, pp 22–25 6. Consultative Committee on Amount of Substance (1998) Report of the fourth meeting. BIPM, Sevres, Paris
G. Price (✉) P.O. Box 57, Menai , NSW 2234, Australia e-mail:
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