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The performance of the protocols was examined in two ways
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by looking at the results of using the two protocols to assess the 'homogeneity' of simulated data
by using the two protocols to assess the homogeneity of FAPAS test materials using the results of measurements carried out between August 1998 and August 1999
Data sets were produced to simulate the results of duplicate analyses of ten test material samples. This simulated data was used to assess the ability of each protocol to discriminate between data sets produced by the analysis of 'sufficiently homogenous' samples and the analysis of heterogeneous samples.
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151 data sets produced, by the analysis of 62 FAPAS test materials between August 1998 and August 1999, were used to assess the likely practical effect of changing the protocol for assessing the homogeneity of test materials. This was achieved by graphically comparing the outcome of each test on each data set.
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An assessment of the quality of the data supplied to the FAPAS Secretariat was also carried out by comparing the means and variances of each series of duplicate analyses within a data set.
A condition for the satisfactory execution of performance assessment schemes, including the Food Analysis Performance Assessment Scheme (FAPAS
), is that the test material should be sufficiently homogenous. In this context, 'sufficiently homogenous'is taken as the variability of the level of analytes in the test material not being large enough to materially effect the outcome of the performance assessment.
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The International Harmonised Protocol for Proficiency Testing of (Chemical) Analytical Laboratories suggests a suitable procedure for establishing the homogeneity of a candidate test material. It also recommends that the estimate of the between-sample standard deviation obtained by the procedure should be less than 0.3 times the target standard deviation for the measurement. However, if the between-sample standard deviation of a candidate test material is marginally below or equal to 0.3 times the target standard deviation then those materials will needlessly fail the homogeneity test.
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The current protocol for the assessment of the homogeneity of a FAPAS
test involves the analysis of ten test material samples under repeatability conditions, with the results of the analyses being exposed to statistical tests to assess the degree of homogeneity of the test material.
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Recent proposals suggest that the test for the homogeneity of test materials should be used to prove heterogeneity (at 95% confidence) before rejecting a test material, rather than confirm homogeneity before accepting a test material.
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A new homogeneity test has been proposed. This protocol is a refinement of the current protocol, including a test for analytical outliers (Cochran's). The homogeneity test within the new protocol combines the F-test and the sufficient homogeneity test of the current protocol into a single test that is designed to 'fail'test materials only when 'heterogeneity' is proven, and as such is designed to avoid the needless rejection of 'sufficiently homogenous'test materials.
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The aim of this study is to compare the results of applying the new test and the current test.
<p>Find more about this project and other FSA food safety-related projects at the <a href="http://www.food.gov.uk/science/research/" target="_blank">Food Standards Agency Research webpage</a>.