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The Horwitz ratio (HorRat) is a normalized performance parameter indicating the acceptability of methods of analysis with respect to among-laboratory precision (reproducibility). It is the ratio of the observed relative standard deviation among laboratories calculated from the actual performance data, RSDR (%), to the corresponding predicted relative standard deviation calculated from the Horwitz equation PRSDR (%) = 2C0.15, where C is the concentration found or added, expressed as a mass fraction. It is more or less independent of analyte, matrix, method, and time of publication (as a surrogate for the state of the art of analytical chemistry). It is now one of the acceptability criteria for many of the recently adopted chemical methods of analysis of AOAC INTERNATIONAL, the European Union, and other European organizations dealing with food analysis (e.g., European Committee for Standardization and Nordic Analytical Committee). The origin and applications of the formula are described. Consistent deviations from the ratio on the low side (values <0.5) may indicate unreported averaging or excellent training and experience; consistent deviations on the high side (values >2) may indicate inhomogeneity of the test samples, need for further method optimization or training, operating below the limit of determination, or an unsatisfactory method.

Although the Association of Official Analytical Chemists (AOAC) has been evaluating and approving methods of analysis for almost 100 years, there is practically no discussion in the Journal of the Association of Official Analytical Chemists of the criteria for determining which methods should be approved for regulatory use. These decisions are usually made on the basis of a method's performance in interlaboratory collaborative studies.John Mandel of the National Bureau of Standards, in his 1981 Shewhart medal address (/), pointed out that the basic objective of conducting interlaboratory tests is not to detect the known statistically significant differences among laboratories: "The real aim is to achieve the practical interchangeability of test results." Interlaboratory tests are conducted to determine how much allowance must be made for variability among laboratories in order to make the values interchangeable.An irreducible difference exists between supposedly identical measurements made in different laboratories. This point was recently demonstrated by a group of New Zealand government laboratories in attempting to minimize the discrepancies in values for blood alcohol between laboratories. The laboratories went to great pains to discover every source of error, even to the extreme of moving analysts from one laboratory to another. They

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