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 the performances of the exposure tool and the second exposure tool based on the comparisons.