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Accreditation and Quality Assurance: Journal for Quality, Comparability and Reliability in Chemical Measurement (v.6, #8)
A life cycle approach to method management by D. G. Holcombe; Stephen L. R. Ellison (pp. 340-345).
This paper describes a life cycle model for method management, using existing published models for software life cycles as a basis. A direct analogy is drawn between the processes in method life cycles and those established for software development, and the constituent parts of the processes that occur in the method life cycle are explored in some detail. It is concluded that a life cycle model can usefully be applied to method management and the main principles are summarised. An example is included to illustrate how life cycle principles can be applied to a real situation.
Keywords: Keywords Life cycle; Method; Validation; Management
Compliance of analytical results with regulatory or specification limits: a probabilistic approach by D. Brynn Hibbert (pp. 346-351).
The practice of determining and quoting an expanded uncertainty with a measurement result allows interpretation of results in relation to compliance limits. Against a fixed limit, the probability that the population mean of the measurement is, in fact, over the limit may be readily calculated, and may be more useful than the observation that the probability is greater or smaller than an arbitrary value (e.g. 95%). When the limit itself is uncertain, it is possible to combine probabilities of the limit and measurement to determine the probability that the given measurement is over the limit. This probability, for an upper compliance limit, is derived from the cumulative probability that a possible measurement value (x M ) is over a given value (X, i.e. Pr(x M > X)) and the probability density function that the compliance limit (x L ) is the same or lower value, f(X > x L ). Integration over all values for X provides the probability that the measurement is over the true compliance limit. A MATLAB script is provided to calculate this probability. A number of examples are assessed using this probability.
Keywords: Keywords Compliance decisions; Regulatory limits; Specification limits; Measurement uncertainty; Probability of compliance
Measurement uncertainty distributions and uncertainty propagation by the simulation approach by P. Tarapčík; Ján Labuda; Blandine Fourest; Viliam Pätoprstý (pp. 352-359).
A complete and accurate evaluation of measurement uncertainty requires the knowledge of the uncertainty distributions. The latter are rarely determined or verified experimentally, and hence up to now only crude estimates or assumptions based on intuition have been used. The simulation of experimental results is readily accessible and provides a more relia-ble solution to this problem. When using an appropriate model of measurement and after determina-tion of input value parameters by present state-of-the-art techniques, simulation data supply reliable information about the distribution of the output results of a complex measurement. The method permits simple variation of preposition and therefore ready analysis of various features influencing the measurement of uncertainty intervals. In the paper we described examples of such evaluations related to the preparation of certified reference materials, where there is excellent agreement between the traditional and simulation approaches. And evaluation of more complex measurements of diffusion coefficients by the open capillary method, where uncertainty of the simulated result is more realistic than the re-sult from the traditional error method due to non-linearity and probably Cauchy distribution in some steps.
Keywords: Keywords Measurement; Uncertainties; Chemical analysis; Distribution law; Monte Carlo simulation
Calibration lines passing through the origin with errors in both axes by Vaclav Synek (pp. 360-367).
Linear regression of calibration lines passing through the origin was investigated for three models of y-direction random errors: normally distributed errors with an invariable standard deviation (SD) and log normally and normally distributed errors with an invariable relative standard deviation (RSD). The weighted (weighting factor is x 2 i ), geometric and arithmetic means of the ratios y i /x i estimate the calibration slope for these models, respectively. Regression of the calibration lines with errors in both directions was also studied. The x-direction errors were assumed to be normally distributed random errors with either an invariable SD or invariable RSD, both combined with a constant relative systematic error. The random errors disperse the true, unknown x-values about the plotted, demanded x-values, which are shifted by the constant relative systematic error. The systematic error biases the slope estimate while the random errors do not. They only increase automatically the slope estimate uncertainty, in which the uncertainty component reflecting the range of the possible values of the systematic error must be additionally included.
Keywords: Keywords Calibration; Heteroscedastic data; Ordinary least squares method; Uncertainty of measurement; x-direction error
Experimental sensitivity analysis applied to sample preparation uncertainties: are ruggedness tests enough for measurement uncertainty estimates? by John R. Cowles; Simon Daily; S. L. R. Ellison; William A. Hardcastle; Carole Williams (pp. 368-371).
It has been suggested that typical ruggedness tests might lead directly to uncertainty estimates. This assertion is tested using simple experimental studies of uncertainties associated with sample grinding and oven-drying operations. The results are used to predict the outcome of typical ruggedness tests on the same systems. It is concluded that uncertainty estimation from ruggedness tests is appropriate only where a strong effect can be observed. Since current practice in ruggedness testing is predisposed to confirming insignificance, typical ruggedness tests are not likely to lead to reliable uncertainty estimates; instead, lack of statistical significance in ruggedness tests is better interpreted as reason to leave an effect out of the uncertainty budget. Only where the ruggedness study is modified in order to achieve statistically significant change is it useful for uncertainty estimation.
Keywords: Keywords Measurement uncertainty; Ruggedness tests; Sample pre-treatment; Moisture; Grinding
Measurement uncertainty in microbiological cultivation methods by R. M. Niemi; Seppo I. Niemelä (pp. 372-375).
Microbiological analyses are carried out on clinical, food, feed and environmental samples. The aims of the analyses are diagnostic or estimation of the safety or the quality of the sample. Important decisions are made on the basis of microbiological analyses. Little attention, however, is paid to the uncertainty of measurement of microbiological analyses. In microbiological cultivation techniques the result is obtained by counting individual objects. The normally low number of counted objects strongly affects the result of the analysis and its uncertainty. Because of the importance of the particle statistical variation to the uncertainty, the approaches developed for chemical analyses are not directly applicable to microbiology. This paper discusses microbial analyses and describes a novel guidance document for the estimation of measurement uncertainty in culturing methods [1].
Keywords: Keywords Microbiology; Cultivation methods; Measurement uncertainty
Plans for implementation of a quality system in the control laboratory of the Romanian National Medicines Agency by I.-L. Popescu; H. Y. Aboul-Enein; D. V. Napradean; G. L. Radu; I. G. Tanase (pp. 376-378).
This paper describes the practical implementation of a quality system in the control laboratory of the National Medicines Agency, Romania, the main aim being the alignment of the requirements of the control of drugs in Romania with European Union standards. Activity in this field is relatively new in Romania, however, it is of great importance to Romania’s compliance with international standards.
Keywords: Keywords Standards; Quality system; Drug control laboratory
National External Quality Assessment Schemes for microbiology, parasitology, and virology in Europe by Tania Crucitti (pp. 379-381).
Quality assurance is an important aspect of laboratory management. One of the activities involved is the participation in external quality assessment (EQA) schemes by the clinical laboratory. These EQA schemes should be organised according to well-defined guidelines, such as the ESO/IEC Guide 43. The present work provides an inventory of the EQA schemes organised in Europe. The schemes are related to the fields of bacteriology, parasitology and virology. For each field various surveys have been organised. Data such as the number of participants, frequency of survey and number of samples in a survey are presented. The nature and way of manufacturing the control material is specified.
Keywords: Keywords External Quality Assessment Schemes; External quality control; Microbiology laboratory