Comparability of measurement results

The foregoing discussion identifies the need for comparability and its achievement by metrological traceability. Comparability of measurement results is conveniently defined in terms of metrological traceability. If two results are traceable to the same stated metrological reference, then they must be comparable. Please note that in common speech comparable often means about the same magnitude, but this is not the case here. In the laboratory comparability of measurement results means simply that the results can be compared. The outcome of the comparison, whether the results are considered near enough to be equivalent, is not a factor in comparability here. 

However the chemical community has not been dormant in the period since the Convention of the Meter came into being. A vast range of analytical methods has been developed with ever increasing sensitivity and selectivity. Comparability of measurement results has been achieved in many sectors by the use of collabora-tively studied methods, reference materials, check samples and proficiency testing schemes. It is only recently that steps have been taken to develop traceability to international standards. In developing this traceability much can be learnt from the way traceability has developed for physical measurements. 

The object of traceability is to enable comparability of measurement results. 

The importance of using common reference scales has been recognised for centuries. For example, in England, King John introduced consistent measures throughout the land in 1215. Other countries also had their own measurements scales standards. Many city museums show the standard measures used for trade within the city or local state. As trade widened so did the need for comparability of measurement results and the use of common units widened. The many different measurements scales were harmonised with the introduction of the metric system and the SI units under the Convention of the Metre signed in 1875. An excellent summary of the historical development of units of measurement is given in the NBS Special Publication 420 [1], Under the Convention of the Metre a hierarchical chain of national and international measurement standards has been developed for the measurement of most of physical quantities. 

Whilst traceability is necessary to achieve comparability of measurement results, it is, of course, not the only requirement of good measurement practice and thus needs to be considered as just one part of the measurement process (2). 

Communautaire. Accreditation aims at the reliability of measurement results and enables laboratories to demonstrate technical competence to their customers. Metrological concepts in food science have become a widely discussed topic over the last few years. Representing the top of the international measurement infrastructure, the National Metrology Institutes (NMIs) have the mandate to disseminate the best practice and measurement capabilities to the beld laboratories (FLs) in their countries. The European and international measurement infrastructure needs high-quality tools to assess the reliability and comparability of measurements results in view of the implementation of EU and international policies regarding internal market activities as well as health and consumer protection issues. 

At the national level, National Metrology Institutes (NMIs) maintain measurement standards and capabilities and provide services in order to develop their national metrology infrastructures so as to underpin and facilitate the comparability of measurement results. 

Description For each analytical measurement, it should be possible to relate the result to an international standard or unit (e.g., international system of units -SI) through an unbroken chain of calibrations [27]. An international standard can also be realized by an internationally accepted reference material, as is the case for some 5-values (e.g., NIST SRM 951 for 5 B). Traceability allows comparability of measurement results. 

Definition Comparability of measurement results that are metrologically traceable to the same reference. 

Metrological attestation as the procedure that ensures reproducibility and comparability of the results of measurements is specified in GOST 8.010-90 State System of Measurements. Procedures of Measurements Implementation didn t find wide use in NDT. In airspace industry, railway and naval transport the requirements of approval of test procedures is in force more than 20 years. In chemical and oil-chemical sectors the similar requirements were less explicit. In some industries, for example in building GOST 8.010-90 was not put into account. 

Figure 8.53 gives measured values of the radiant temperature, which means Ty - T, caused by an electrical heating panel. The effective surface temperature of the panel can be estimated from the curves, then used to calculate the temperature of a thermometer bulb at a few other places. These results can be compared to measured results. 

The principle of calibration is to compare the measurement result of the manometer to be calibrated to that of the measurement result of the reference... 

This is the essence of AC impedance techniques the cell is replaced by a suitable model system, in which the properties of the interface and the electrolyte are represented by appropriate electrical analogues and the impedance of the cell is then measured over as wide a frequency range as possible. By comparing the measured results with values calculated from the model system, we can evaluate both the suitability of our model and the values of the parameters required. 

Measurement bias is determined by comparing the mean of measurement results obtained for a reference material, using the method being validated, with the assigned property values for that reference material. The number of replicate analyses required (n) depends on the precision of the method (.s ) and the level of bias (8) that needs to be detected [12]. A useful approximation is shown in the following equation ... 

If the result reported for a measurement represents the average of a number of measurements, rather than a single measurement, the standard deviation of the mean (s/ Jn) is the correct estimate of the standard uncertainty. In other words, compared to the uncertainty for a single result, the standard uncertainty for an average result is reduced by a factor of >Jn (where n is the number of measurement results that were averaged to obtain the reported result). 

Fukai, R., B. Oregioni, and D. Vas. 1978. Interlaboratory comparability of measurements of trace elements in marine organisms results of intercalibration exercise on oyster homogenate. Ocean. Acta 1 391-396. 

In this section, the numerical solutions of the MINLP-model and of the MILP-model as presented in Sections 7.4 and 7.5 are compared with respect to their solution quality (measured by the objective values) and the required solution effort (measured by the computing time). In order to compare the MILP-solution with the MINLP-solution, the optimized values for the start times of polymerizations tn, the recipe assignments W, and the total holdups Mnr are inserted into the MINLP-model and the objective is calculated. To guarantee comparability of the results, the models were stated with identical initial conditions, namely t° = 0, = 2 Vk, pf = 0 Vs, and ra = 0.4 Vs (i.e., the variables defined at the beginning of the corresponding time axes are fixed to the indicated values). For the algorithmic solution procedure, all variables were initialized by 1 (i.e., the search for optimal values starts at values of 1 ), and none of the solvers was specifically customized. 

Preparing and measuring a series of standard solutions and plotting the standard curve is the usual process of calibration when the response of a device to a standard is not known in advance (see answer to number 8). In other words, a series of standards establishes the result that is not known in advance. With such a calibration in effect, the operator can then measure samples by comparing the measured result for the sample to the standards results. 

If the results of analytical chemistry are used as the bases for decisions, a client will frequently compare the measurement result with some other value. In process control, a company will have release specifications that must be... 

Science, technology, and commerce require rapidly rising numbers and types of measurements that for good reasons can be trusted [1-4]. Worldwide acceptance of measurement results requires reliable, traceable, and comparable measurements for reduction of costs, efficient production processes, subsequent use of measurement data, realization of fair-trade conditions, and for internationally recognized and accepted laboratory accreditations. Physical measurements made in accor-... 

To achieve this goal, comparability of chemical measurements based on traceability to recognized standards and hence on thorough knowledge of uncertainty, must be established in analogy to the way in which the validity of measurement results is ensured in metrology in general. This task mainly consists of two parts ... 

Traceability to a common unit, or to a common measurement scale, makes the results of measurements comparable, regardless of their magnitude. The very -and only - reason for the existence of the requirement traceability is to enable us to compare our measurement results (i.e. achieving comparability) measurements of the same measurand, carried out at different places and/or different times, yield concordant results (meaning that they fall within each other s stated uncertainty). It is emphasized again that traceability need not to be to (values of) SI units but can be to values of other (including arbitrary) units with which measurement scales can be built. As Fig. 2 shows, a measurement scale can be constructed from a calibration curve based on a number of values carried by commonly agreed reference materials (in this case 5) [2],... 

Abstract In this article the role of reference materials is confined to chemical measurements only. Recognized reference materials are one of the tools to obtain comparability of analytical results. Recognition demands confidence in the reference materials and in the reference material producers. A reference material producer is a technical competent body that is fully responsible for the certified or other property values of the reference material. The analyte has to be specified in relation to the selectivity of analytical procedure. The full range of reference materials can be presented as a three-dimensional space of the coordinates analyte,... 

It is shown how reference values with a demonstrated traceability and demonstrated uncertainty (according to ISO-GUM) are disseminated by IRMM to field (and other) laboratories by means of appropriately prepared test samples. The reference values are established using internationally recognized measurement capabilities and are demonstrated to be equivalent at the international level. The uncertainties are the end-product of an exhaustive evaluation process, yet, the resulting combined and expanded uncertainty are sufficiently small for the intended use of the result (i.e. to be smaller than the expected interlaboratory spread of the participants measurement results). After having measured these samples, the participating field (and other) laboratories can compare their measurement results with these reference values, which are released after they have submitted their own results. Both the reference value and its measurement uncertainty as well as the participants values with their declared measurement uncertainty are displayed in simple, comprehensive pictures. 

Abstract For ensuring the traceability and uniformity of measurement results, the main objectives of national metrology programmes in chemistry are to calibrate and verify measuring instruments, to evaluate the uncertainty of measurement results and to intercompare the analytical results, etc. The concept of traceability has developed recently in chemical measurements, thus, an attempt to implement the principles of metrological traceability especially by appropriateness calibration using composition certified reference materials (CRMs) is underlined. Interlaboratory comparisons are also a useful response to the need for comparable results. The paper presents some aspects and practices in the field of spec-... 

When the analytical laboratory is not responsible for sampling, the quality management system often does not even take these weak links in the analytical process into account. Furthermore, if sample preparation (extraction, cleanup, etc.) has not been carried out carefully, even the most advanced, quality-controlled analytical instruments and sophisticated computer techniques cannot prevent the results of the analysis from being called into question. Finally, unless the interpretation and evaluation of results are underpinned by solid statistical data, the significance of these results is unclear, which in turn greatly undermines their merit. We therefore believe that quality control and quality assurance should involve all the steps of chemical analysis as an integral process, of which the validation of the analytical methods is merely one step, albeit an important one. In laboratory practice, quality criteria should address the rationality of the sampling plan, validation of methods, instruments and laboratory procedures, the reliability of identifications, the accuracy and precision of measured concentrations, and the comparability of laboratory results with relevant information produced earlier or elsewhere. 

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