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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.13, #1)
Lectures for chemists on statistics. I. Belief, probability, frequency, and statistics: decision making in a floating world by Günther Meinrath (pp. 3-9).
A brief account on the development of probability theory and statistics is given. These subjects may help the chemist to interpret the information content of his experimental data. Their source and validity, however, is often unclear. A series of papers, of which this is the first, intends to present a clearer view by illustrating the origin of the data-analysis tools. The major figures in this field are mentioned, their achievements presented and the mutual links outlined. The intention is the provision of a common basis for the discussion of statistical aspects relevant to metrology in chemistry. The chemist should see probability theory and statistics as tools to assist in the daily practice of data interpretation to arrive at an objective conclusion.
Keywords: Probability theory; Statistics; Determinism; Pearson; Neyman; Fisher
Estimation of measurement uncertainty in organic analysis: two practical approaches by Penka Shegunova; Ofelia Bercaru; Berit Sejerøe-Olsen (pp. 11-18).
Estimation of measurement uncertainty has become a more regularly performed part of the whole analytical process. However, there is still much on-going discussion in the scientific community about ways of building up the uncertainty budget. This study describes two approaches for estimation of measurement uncertainty in organic analysis: one which can be used for single sets of measurements and the other based on validation studies. In both cases the main contributions to the uncertainty are presented and discussed for the analysis of PCBs in mussel tissue, but the approaches can be extended to other organic pollutants in environmental/food samples. The main contributions to the uncertainty budget arise from calibration, sample preparation, and GC–MS measurements. A comparison of the relevant sources and their contributions to the expanded uncertainty is presented.
Keywords: Uncertainty; PCBs; Mussel tissue; Reference materials
Comparison of models for estimation of measurement uncertainties. Determination of BOD5 in wastewaters using a manometric method by Maria Ascensão Trancoso; Sandra Catarina S. Calisto (pp. 19-28).
Biochemical oxygen demand after 5 days (BOD5) is a bioassay frequently used to determine the polluting strength of wastewaters. It quantifies the mass concentration of dissolved O2 consumed during an incubation period of 5 days at 20 °C, in the dark, by biological oxidation of organic and/or inorganic matter in wastewater. In this work, the BOD5 measurand is reported as γ(O2). Reporting analytical results with estimation of uncertainty is an ISO/IEC 17025:2005 requirement and allows results to be compared and their fitness for purpose to be demonstrated. The γ(O2) uncertainty was estimated using two models: the individual components’ model (model 1) and the quality-control-criteria model in which the trueness component is calculated using the equation for recovery from CRMs (model 2.A) and the standard deviation of all recoveries (model 2.B). Model 1 provides a realistic strategy for estimation of γ(O2) uncertainty and for assessment of performance. Models 2.A and 2.B are expeditious and simple procedures for the 100–800 mg L−1 range. When γ(O2) < 100 mg L−1 model 1 must be used because with both Models 2 uncertainty was underestimated, more drastically close to the limit of quantification.
Keywords: Biochemical oxygen demand; Manometric method; Uncertainty
Measurement uncertainty evaluation for a non-negative measurand: an alternative to limit of detection (pp. 29-32).
The interpretation and reporting the results of measurements on materials where the concentration of the analyte is close to or may even be zero has been the subject of much discussion with the use of such concepts as limit of detection (LOD) and limit of quantification (LOQ). While these concepts have taken into account the measurement uncertainty, they have not utilised the fact that the value of the measurand, i.e., the concentration, is constrained to be zero or greater. Taking this into account the distribution of values attributable to the measurand can be derived from the probability density function (PDF) that determines the distribution of the observed values. When this PDF is normal the distribution of the values attributable to the measurand is a truncated t distribution with a lower limit of $$ t_L = - x_m /left( {{s mathord{left/ {vphantom {s {sqrt n }}}
ight. kern-
ulldelimiterspace} {sqrt n }}}
ight), $$ re-normalised so that the total probability is one, where x m is the mean of the n observed values and s their standard deviation. When x m much greater than $$ {s mathord{left/ {vphantom {s {sqrt n }}}
ight. kern-
ulldelimiterspace} {sqrt n }} $$ then the distribution reverts to the unmodified t distribution. The probability that the value of the measurand is above or below a limit can be calculated directly from this truncated t distribution and the interpretation of the result does not require the use of concepts such as LOD and LOQ. Also it deals with the problem of negative observations.
Keywords: Measurement uncertainty; Limit of detection
The establishment of quality systems in veterinary diagnostic testing laboratories in developing countries: experiences with the FAO/IAEA External Quality Assurance Programme by Axel Colling; Martyn Jeggo; Helder Louvandini; Mamadou Lelenta; Mark Robinson (pp. 33-45).
Quality systems, established to internationally accepted standards, are one mechanism that can assist in evaluations of the sustainability of technology transfer, the proficiency of the user, and the reliability and comparability of data generated, resulting in potential enhancement of laboratory credibility. The means of interpreting existing standards and implementing quality systems in developing country veterinary diagnostic laboratories has become a significant adjunct to the technology transfer element within the Food and Agriculture/ International Atomic Energy Agency, FAO/IAEA programme. The FAO/IAEA External Quality Assurance Programme (EQAP) is given as an example for an initial step towards enhancing the “quality” culture in developing country veterinary laboratories. In 1995 the EQAP began as an effort to assure that test results emanating from laboratories using FAO/IAEA ELISA kits for animal disease diagnosis are valid. For this purpose 15 international external quality-assurance rounds have been performed to date for a variety of animal diseases e.g. Rinderpest, brucellosis, trypanosomosis, and foot-and-mouth disease (FMD). Results indicate that the EQAP is a valuable tool in the assessment of both the results provided by, and use of the ELISA kits provided through, the joint FAO/IAEA programme. Furthermore EQAP can assist laboratory diagnosticians to enhance quality control/quality assurance (QC/QA) procedures for conducting FAO/IAEA ELISAs and to advise on the implementation of similar QC/QA procedures in other laboratory activities. Based on the experiences made during the implementation of the EQAP a proposal for establishing a quality system standard was ratified through the World Organization for Animal Health (OIE) general conference in May 2000. The OIE Standard On Management And Technical Requirements For Laboratories Conducting Tests For Infectious Animal Diseases is based on ISO 17025 and provides a clear formula for establishing quality systems in veterinary diagnostic laboratories world-wide.
Keywords: External quality assurance; Proficiency testing; Quality systems; Developing countries; Diagnostic laboratories; Animal health; ELISA
Implementation of a quality management system in meteorological practice in Serbia by Mica Jovanovic; Jovan Jovanovic (pp. 47-52).
An optional approach in the meteorological service Quality Management System (QMS) based on the case study of the Republic Hydrometeorological Service (MHS) of Serbia is presented and discussed in this paper. The idea is to use the ISO/IEC 17025 standard as a base for its QMS development. Main reasons for such a decision and the elements of the implemented solution are presented and discussed. Meteorological testing processes of the MHS of Serbia and relevant ISO/IEC 17025 scope of accreditation are presented. After establishing competencies in meteorological, environmental, and hydrological testing, the MHS of Serbia will develop an integrative ISO 9001 QMS. The Serbian case study could be taken into consideration as one of the options related to the future World Meteorological Organization Quality Management Framework.
Keywords: Accreditation of meteorological service; ISO/IEC 17025; Quality management system; WMO quality management framework
Report of the 30th meeting of ISO/REMCO
by Adriaan M. H. van der Veen; Ales Fajgelj; Hendrik Emons; Stéphane Sauvage (pp. 53-55).
A comment on: “A forgotten fact about the standard deviation”
by B. D. Hall; R. Willink (pp. 57-58).