DATA-PROCESSING IN THE INTERLABORATORY TEST BY ANALYSIS OF COVARIANCE

The results of an interlaboratory test on the determination of total c holesterol and triglycerides content in standard reference materials ( frozen human blood sera) are discussed. The test's setup included the possibility of estimating the concentration-dependent effects bath bet ween laboratories and within each laboratory (run-to-run effects). The interlaboratory test data have been calculated on the basis of the mo del of analysis of covariance providing simultaneous assessment of qua litative (here, concentration-independent) and quantitative (proportio nal to concentration) effects. Intra- and interlaboratory effects, bot h independent on the concentration of the substances measured and thos e proportional to them, were significantly detected. These effects are interpreted as constant and proportional constituents of the error at various levels: between runs of the analysis within each laboratory; between laboratories; for all laboratories participating in the test. It was shown that with analysis of covariance it is possible to detect the errors where they cannot be detected with the conventional method of the analysis of variance. It is shown that the analysis of covaria nce is extremely useful to develop a recommendation to reduce intra- a nd interlaboratory errors of analysis, as it allows the researchers to assess separately constant and proportional constituents of the error typically coming from different sources.