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Audrey Bluomsohn explained how 'errors' are propagated in various mathematical operations. I would prefer, though, to talk about 'uncertainties'. Although the formulas that Audrey Blumsohn uses are quite correct, the conclusions are not entirely so. Since squares are always positive, the variance of a sum of variances will always be larger than any of the individual variances. To use this simple relation it is most important that the variables are uncorrelated. The second relation that Audrey Blumsohn quotes is an approximation that, however, is most useful. Relations 1 and 2 can be conveniently combined to tackel more complex funtions. A more detailed approch requires partial derivatives. All this is nicely described in a joint publication of IUPAC, IFCC, ISO, BIPM and others. It has become a standard publication and is commonly known as GUM, 'Guide to the interpretation of uncertainties of measurements'. It is highly recommended. The concept of uncertaity, as defined by ISO and expanded in this publication, simplifies much of the problems of random and systematic 'errors'. Among other interesting things it contains a very useful spreadsheet template to come closer to a partial derivative approach to estimate the propagation of uncertainies. Another example is that this approach opens an interesting new possibility to define 'detection limit'! Anders Kallner Dept Clin Chem, Karolinska hospital, Stockholm Sweden E-mail address: [log in to unmask] phone: +46 (0)8 5177 49 43 fax:+46 (0)8 5177 28 99