Measure mucertainty.pdf

obiology 116 (2007) 44–51 /locate/ijfoodmicroInternational Journal of Food MicrAssessment of measurement uncertainty for quantitative methods of analysis: Comparative assessment of the precision (uncertainty) of bacterial colony counts Basil Jarvis a, Alan J. Hedges b, Janet E.L. Corry c,? a Ross Biosciences Ltd, Upton Bishop, HR9 7UR, UK b Department of Cellular and Molecular Medicine, University of Bristol, Bristol BS8 1TD, UK c Department of Clinical Veterinary Science, University of Bristol, Langford, BS40 5DU, UK Received 30 June 2006; received in revised form 9 October 2006; accepted 10 December 2006Abstract An interlaboratory trial was made, by three analysts in each of the 19 laboratories, of three International Standards Organisation (ISO) colony count methods for aerobic organisms (ACC), Enterobacteriaceae (ECC) and Escherichia coli (EcCC). All tests were done in duplicate and were further replicated by plating both on culture media supplied by the organisers and on each laboratorys own choice of culture media. In order to avoid any influence of food matrix on the results, the inoculum for each test was a freeze-dried ampoule of a standardised mixed culture. After collation of test results, individual data sets were examined for obvious non-consistency, and colony counts for each individual test were determined both as simple and ‘weighted’ mean values. The derived colony counts were then log10-transformed and examined statistically. Estimates of repeatability and reproducibility for each set of results were derived and used to calculate the parameters for the uncertainty of measurement. Estimated values of the uncertainty of reproducibility and repeatability for the ACC ranged, respectively, from 9.3 to 12.1% and 2.0 to 3.9% of the mean log10 colony count, depending on the specific culture media, the method of deriving the mean count and the statistical procedure employed. Similarly, estimates of the uncertainty of reproducibility and repeatability for th