1. A method comprising: selecting an optical model for an exposure tool; selecting a second optical model for a second exposure tool; simulating operation of the exposure tool using the optical model and an individual mask error model of a mask to produce first simulated results; simulating operation of the second exposure tool using the second optical model and the individual mask error model to produce second simulated results; performing a comparison of the first simulated results to a design target; performing a comparison of the second simulated results to the design target; and comparing performances of the exposure tool and the second exposure tool based on the comparisons, wherein the mask is physically manufactured based on the design target, and the individual mask error model is created based on an inspection of the manufactured mask.