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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.13, #11)
Operating ‘cost’ characteristics in sampling by variable and attribute by Leslie R. Pendrill (pp. 619-631).
A discussion in economic terms of common rules in conformity assessment based on measurement is given. The present work extends tools of sampling when using inspection by variable and attribute, such as the setting of acceptance quality limits (AQL) and limiting quality limits (LQL), by including an economic decision theory approach, leading, amongst other things, to a new ‘cost’ curve as a complement to the traditional operating characteristic curves of statistical significance testing. The methodology is of general applicability but is illustrated in the present work in the simple case of homogeneously pre-packaged goods priced linearly with the amount of content. The relation to the optimised uncertainty methodology is explained. Optimum strategies for the supplier are illustrated in terms of minimising production and testing costs, while at the same time maintaining satisfactory levels of customer satisfaction.
Keywords: Acceptance quality limits; Limiting quality limits; Economic decision theory; Cost characteristics
Principles of the EURACHEM/CITAC guide “Use of uncertainty information in compliance assessment” by Alex Williams (pp. 633-638).
Many, possibly most, analytical measurements are carried out to assess compliance with a specification or a regulation, for example in the control contaminants in food or the detection of performance enhancing substances in sport. When making an assessment of compliance the presence of unavoidable measurement uncertainty introduces the risk of making incorrect decisions, that is of accepting a batch of material which is outside the specification or rejecting one that is within. This often leads to controversy over whether or not the compliance decision is correct. How to make reliable assessment decisions is described in the EURACHEM/CITAC Guide “Use of uncertainty information in compliance assessment”. The key is the use of decision rules that lead to an unambiguous interpretation of the measurement result and its uncertainty. These decision rules need to be designed to ensure that requirements of the specification or regulation are met and that the risk of making an incorrect decision is acceptable. Ideally they should form part of the specification or regulation.
Keywords: Measurement uncertainty; Assessment of compliance
Setting up a decision rule from estimated uncertainty: emission limit value for PCDD and PCDF incineration plants in Wallonia, Belgium by B. Bergmans; F. Idczak; P. Maetz; J. Nicolas; S. Petitjean (pp. 639-644).
Complex analytical procedures are often required to prove the non-compliance with a specific legislation. In the case of a small overlap of the limit, integration of the method uncertainty in the decision-making process is essential. The decision rule proposed in Wallonia, Belgium, for the non-compliance of waste incineration plants with the EU limit value for PCDD and PCDF emissions is presented. The method uncertainty was estimated annually over 6 years from duplicate measurements using two top-down approaches. Depending on the congener, the standard uncertainty varies from 30 to 85%, with a good correlation between calculations. The analytical contribution was estimated using a bottom-up evaluation. The impact of the sampling step was deduced from the whole estimation and represents more than 80% of the total uncertainty budget. No optimisation is foreseen at this time because of practical field constraints. Based on the average fraction of each congener, the uncertainty associated with the measurement result has been established and shows a high stability over the years. Using this value, a guard band has been calculated and will be proposed to the regulatory body.
Keywords: Uncertainty; Top-down; Bottom-up; Decision-making; Dioxin
Usage of the uncertainty of measurement by accredited calibration laboratories when stating compliance by Martin Czaske (pp. 645-651).
First, concepts to state compliance with specifications are presented when taking into account the uncertainty of measurement. Then, the methods used by accredited calibration laboratories, especially within the Deutscher Kalibrierdienst (DKD), are introduced and compared with these methods and concepts. Compliance can only be stated with some probability (risk), which can be calculated by integration, if the measurement results and its probability density functions are known. A scheme for the calculation of the risks by the Monte Carlo method leads to zones of correct and false acceptance, and correct and false rejection. The user of the device under test or calibration has to decide which risk is acceptable. A statement of compliance should only be made if the measurement capability index is not smaller than about 2. The correspondence of probabilities of compliance and guard bands is shown. The rules of ISO 14253-1 and DKD-5 are equivalent to the usage of a guard band of special width. Finally, the performance criteria for comparisons in calibration are treated briefly. Their use has some similarity to the decisions made on compliance.
Keywords: Compliance; Measurement; Calibration; Risk; Comparison measurements
About considering both false negative and false-positive errors when assessing compliance and non-compliance with reference values given in compositional specifications and statutory limits by E. Desimoni; B. Brunetti; A. Clerici (pp. 653-662).
Taking account of the uncertainty in measurement is essential when making compliance decisions, but how to do it is problematic. This paper considers two types of compliance decision rule. One set of rules is that described in publications from a number of national and international bodies while the second set is one developed by this laboratory. The effect of using of the two sets of rules on the outcome of compliance decisions associated with simulated measurement data and real measurement data is evaluated. The strengths and weaknesses of the different approaches are also discussed and it is concluded that the best approach depends on the type of measurement.
Keywords: Limit of detection; False-positive and false-negative errors; Non-compliance assessment; Limit of non-compliance
Risk of erroneously deciding conformity of measuring instruments by Karina Weißensee; Olaf Kühn; Gerhard Linß; Klaus-Dieter Sommer (pp. 663-669).
Due to the unavoidable measurement uncertainty associated with every measurement and calibration result, conformity testing always implies the risk of taking erroneous decisions. Whereas end-users of measuring instruments are concerned mainly with instruments that have successfully passed a conformity test but actually indicate outside their specifications, instrument manufacturers are interested primarily in a low fail-error probability. In this paper, a straightforward Bayesian approach to evaluating pass- and fail-error probabilities in deciding conformity of measuring instruments is presented. It is based mainly on the expression of both knowledge of the respective calibration result of a measurement and the distribution of error values of the instrument population by means of appropriate probability distributions. The risk-calculating approach is explained using the example of conformity testing of a precision scale.
Keywords: Measurement uncertainty; Conformity testing; Pass- and fail-error probability
Review of Symposium “New developments in measurement uncertainty in chemical analysis”
by Leslie R. Pendrill (pp. 671-674).