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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.9, #8)
Comments on “Cofino statistics” by Tom Fearn (pp. 441-444).
Cofino et al. have put forward a method for the analysis of data from interlaboratory studies. The method uses ideas and notation from quantum chemistry, and is quite sophisticated. A critical examination shows that it has some relationship with both kernel density estimation and robust estimation methods. The quantitative results it provides are not founded on any statistical probability model, and it is not at all clear how the variance in particular is to be interpreted. There is no clear argument for adopting Cofino statistics in preference to simple problem-free methods that are at least as effective in estimating a concensus value.
The assessment of compliance with legal limits. Part 2. How to consider both type I and type II errors when working in the concentration domain by E. Desimoni; B. Brunetti; R. Cattaneo (pp. 445-449).
Considering the uncertainty of measurement when assessing compliance with reference values given in compositional specifications and statutory limits is still a controversial matter. In theory, assessing compliance requires considering both type I (false positive) and type II (false negative) errors. The more the concentration of the analyte in the sample under investigation is close to the allowed concentration limit, the more critical it is to consider both types of errors. This paper describes how this could be done. The matter is discussed in the light of the most recent literature information.
Keywords: Uncertainty of measurement; Legal limits; Type I and type II errors; Maximum concentration value and minimum concentration value; Minimum detectable inadmissible concentration and maximum detectable admissible concentration
Dimethylsulfone as a universal standard for analysis of organics by QNMR by Robert J. Wells; Julie Cheung; James M. Hook (pp. 450-456).
Quantitative nuclear magnetic resonance (QNMR) is a primary analytical method which allows the use of a standard not directly related to the target analyte. This could allow the introduction of a single universal reference material (URM) to which all other organic standard reference materials/certified reference materials may be compared. Dimethylsulfone is a simple substance that fulfils all criteria necessary as the URM to which the QNMR analysis of all other organic substances may be referenced and hence we propose this as a candidate for this purpose.
Keywords: Quantitative NMR; Dimethylsulfone; Reference materials; Primary methods
The statistical basis of standardization designs for diagnostic assays by Andrea Konnert; Christoph Berding (pp. 457-463).
Accuracy and long-term quality of laboratory diagnostic assays depend critically on the standardization process. In this note we review statistically typical procedures and designs used in standardization. Issues of practical relevance such as the number of systems, runs, and replicates to be involved in the standardization design, and quality control aspects, are addressed.
Keywords: Standardization design; Value assignment; Analysis of variance; Outlier analysis
Planning and combining of isochronous stability studies of CRMs by T. P. J. Linsinger; A. M. H. van der Veen; B. M. Gawlik; J. Pauwels; A. Lamberty (pp. 464-472).
The first part of this paper discusses some critical factors in planning and evaluation of isochronous stability studies. Recommendations on the number of time-points, number of items, and number of replicates to be measured on each unit are given. In general, more than two time-points should be used. The effect of batch heterogeneity can be eliminated by spreading the results over more units. An approach for combining two or more isochronous stability studies is developed in the second part. This approach includes a potential correction of the earlier stability study, pooling of data of the stability studies, and an uncertainty evaluation taking into consideration the uncertainty of the initial correction. The statistical basis for this approach is developed and equations for evaluation of the uncertainty of stability of the combined study are given.
Keywords: Reference material; CRM; Stability study; Stability monitoring; Isochronous measurements; Uncertainty of reference materials
Correlations between the activities of a gamma-ray emitter calculated from different peaks in the spectrum by Denis Glavič-Cindro; Matjaž Korun; Branko Vodenik (pp. 473-477).
A model for calculating correlations between the activities of the same gamma-ray emitter calculated from different peaks in its spectrum is presented. The correlation coefficients can be expressed by the relative uncertainties of the input quantities. The use of this model in the calculation of the mean activity prevents the calculation of an excessively small uncertainty of the mean, since averaging of correlated uncertainty components is avoided.
Keywords: Gamma-ray spectrometry; Uncertainty budget; Correlations
A strategy for a national metrology institute to create a cost effective distributed metrology infrastructure for chemical measurements by Philip Taylor; Ivo Leito; Nineta Majcen; Algirdas Galdikas; Emilia Vassileva; Steluta Duta; Ewa Bulska (pp. 478-484).
A sound strategy for a national metrology institute (NMI) is proposed, describing how to set up an metrology infrastructure for chemical measurements. A national measurement infrastructure is defined as a collection of various measurement services (testing, calibration and reference laboratories) and the communication between these services. For clarity, in this paper the distributed metrology infrastructure covers those organisations that are involved in disseminating measurement traceability (i.e. the national metrology institute and the reference laboratories acting as national reference standard holders).The strategy aims at a proper support of sectoral field laboratories. It is based on a distributed metrology system. Such a system is composed of clearly identified “national reference standard holders” for particular measurement services (e.g. for a particular analyte in a particular matrix) co-ordinated via an NMI. Such “national reference standard holders”, appointed by the NMI, represent the best measurement capability inside the country, and their appointment is based on demonstrated measurement competence. They receive support (e.g. under contract) from the NMI to fulfil this role. They have the obligation to demonstrate their measurement capabilities on a regular basis and in a publicly open and transparent way.In particular and carefully selected cases, the NMI itself can and should act as national reference standard holder. The NMI should particularly devote a large part of its resources to cross-sectoral knowledge transfer, to advice and co-ordination. This can be achieved by participating in teaching/training, by supporting the accreditation, by being involved in advising governmental bodies in authorisation of laboratories and by assisting in the implementation of legislation.As a consequence, only when values produced at the NMI (or one of its designated national reference standard holders) are disseminated to field laboratories (e.g. for CRMs or as a calibration service) will it be necessary to have the NMI measurement capability recognised under the CIPM-MRA system.Such a distributed system requires an efficient communication tool between the three stakeholders concerned: the NMI, the national reference standard holder and the end users. The latter not only include the field laboratories, but also governmental bodies and the national accreditation body.
Keywords: Metrology infrastructure; National metrology institutes; MiC; Metrology in chemistry
Case study of the effectiveness of a distributed metrology infrastructure for chemical measurements: supporting measurements for the regulated content of proteins in wheat in Slovenia by Nineta Majcen; Philip Taylor (pp. 485-487).
This paper describes a demonstrator case study of how a distributed metrology institute can bring value to the measurement infrastructure. The example is taken from the food measurement area, where the Metrology Institute of the Republic of Slovenia (MIRS) has set up a collaboration with the Slovenian National Food Reference Laboratory (NFRL) to act as the holder of the reference standard for the measurement of the content of protein in wheat. The responsibilities of the different parties are described; examples of the joint activities are given. The benefits of the co-operation are described in detail.
Keywords: Metrological infrastructure; Measurements in chemistry; Quality of food
Evaluation of accuracy and precision in an inter-laboratory comparison of total mercury in hair by Udai S. Gill (pp. 488-492).
Three round robins, involving 22 laboratories from eight countries, were conducted at six month intervals. Each of the participating laboratories analysed four samples of NIES-13 in three subsequent round robins during July 2000–October 2001. The objective of the current study was to determine accuracy and precision among participants for the analysis of total mercury. Both accuracy and precision for total mercury determinations in powder hair samples were good. The median within-laboratory (within-run) CVs ranged from 3.1 to 3.9% for four CRM samples. Most laboratories showed a proportional bias relative to the consensus mean of up to 16%. Two laboratories reported results that, on average, were almost 30% higher than the consensus mean value. Significant among-laboratory imprecision was found in the present study. Improvements are needed to reduce the analytical imprecision in a few laboratories, and attention must be focused on calibration issues and methodology-related problems.
Keywords: Quality Assurance; Interlaboratory; Proficiency testing; Accuracy and precision
Validation steps for traceability of linear calibrated chemical measurements by L. Brüggemann; R. Wennrich (pp. 493-498).
Based on the new draft of the EURACHEM/CITAC Guide “Traceability in Chemical Measurement”, this publication describes how traceability can be achieved for chemical measurements using a linear calibration function. Traceability can be accomplished without larger expenditure, if the measurement is calibrated on the basis of appropriate reference standards and the linear regression employed is selected and validated statistically in a suitable form. The determination of nickel in aqua regia eluates of sediment samples, employed for an ICP-OES measurement, is used as a practical illustration of this approach.
Keywords: Traceability; Linear calibration; Method validation
External proficiency testing exercises on human leukocyte antigen typing in Italy between 1998 and 2002 by Francesca Quintieri; Orsola Pugliese; Mirella Mariani; Manuela Testi; Alessandro Nanni Costa (pp. 499-502).
External proficiency testing exercises are essential tools for measuring the quality of different kinds of performances related to health care and to ensure that they are uniform throughout a region. For many years, the Italian National Transplant Centre and the Italian National Institute of Health have been organizing external proficiency testing exercises for serological and genomic human leukocyte antigen (HLA) typing, following the rules of the European Federation of Immunogenetics. This society provides accreditation for immunogenetical tests to those European countries which want to collaborate on programmes on solid organ and bone marrow transplantation. The results of the Italian external proficiency testing exercises are very satisfactory because of the progressive improvement of the quality of HLA typing seen in Italian laboratories between 1998 and 2002. The attained outcome will certainly guarantee a more reliable service in clinical practice, and will contribute to the process of approaching the highest quality in health care.
Keywords: External proficiency testing exercises; HLA typing; EFI accreditation; Transplantation
Method validation and qualification of instruments for atomic spectrometry by Barbara Pohl (pp. 503-506).
This work describes the analytical procedures for atomic absorption and inductively coupled plasma (ICP) techniques that have to be used in order to obtain a license to sell drugs in the USA. The qualification of atomic absorption spectrometers and ICP instruments is described. The method validation characteristics, e.g., accuracy, precision, linearity, range, detection limits, and quantification are discussed. The time involved and the quality of documentation are pointed out. The consequences for laboratory personnel and operating costs are also discussed.
Proper use of certified reference materials? by Lars Jorhem (pp. 507-508).
Certified reference materials (CRMs) are important quality assurance tools in the laboratory. It is, however, not certain they are always used properly. Several guidelines are available on the use of CRMs, but appear to be mostly unknown. The users thus get very little help on the way to proper use. Many of the problems with CRMs are well known, but that does not mean they have disappeared. Better use of CRMs will require the involvement of producers and analytical societies, e.g. the AOAC. Editors of international analytical journals could have a great influence on how CRM results and other QA data are treated in publications.
Keywords: Certified reference materials; Analytical quality assurance
Accounting for bias in measurement uncertainty estimation by Werner Haesselbarth (pp. 509-514).
Generic issues and current approaches in the evaluation of bias studies with respect to estimation of measurement uncertainty are discussed, focusing on two main scenarios. In the first, for a within-laboratory assessment of a fully developed uncertainty budget, bias studies are carried out to verify the pre-established performance of a measurement procedure, and design corrective actions if necessary. In the second scenario, for an estimation of measurement uncertainty from within-laboratory validation data, bias studies are carried out to “calibrate” the whole measurement procedure and evaluate its performance, more specifically to estimate the uncertainty of measurement by combination of bias and precision estimates.
Some comments on King’s statements concerning measurement uncertainty in doping control
by Adriaan M. H. van der Veen (pp. 515-517).