In this paper, we describe a new method for the separation of tool-induced measurement errors and sample-induced measurement errors. We apply the method to standard overlay target configurations. This method is used to separate the effects of the tool and sample errors in the measured optical intensity profiles and to obtain the best estimate of the correct intensity profile for a given sample geometry. This most accurate profile is then compared to calculated profiles from two different theoretical models. We explain the modeling in some detail when it has not been previously published.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.