Abstract: A method for determining if an undesirable feature on a photomask will adversely affect the performance of the semiconductor integrated circuit device that the mask is being used to create. The method includes inspecting the photomask for undesirable features and analyzing the designed features close to the defects. This analysis is performed on lithographic images that represent the image that is transferred onto the semiconductor wafer by the lithography process. This analysis takes into account the effect of variations that are present in the lithography process. Through knowledge of the effects of variations in mask critical dimension of a feature on the lithographic image of that feature, the analysis results in the assignment of an equivalent critical dimension error to the defect. This equivalent critical dimension error is then compared to the mask critical dimension error specification to determine whether or not the defect will adversely affect the device.