Variability, legal limit

Horwitz points out the universal recognition of irreproducible differences in supposedly identical method results between laboratories. It has even been determined that when the same analyst is moved between laboratories that the variability of results obtained by that analyst increases. One government laboratory study concluded that variability in results could be minimized only if one was to conduct all analyses in a single laboratory. .. by the same analyst . So if we must always have interlaboratory variability how much allowance in results should be regarded as valid - or legally permissible as indicating identical results. What are the practical limits of acceptable variability between methods of analysis - especially for regulatory purposes. 

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