Optical lithography, since many years the workhorse in manufacturing of integrated circuits, is being pushed to its limits. The extension of photolithography has been made possible by improvements in resist schemes and by resolution enhancement techniques. Although the resolution capabilities are available, maintaining CD-control will be one of the major challenges for photolithography engineers in the future. Traditionally, focus and exposure latitude are the principal criteria used in lithography. In this paper, we use an alternative method to quantify the performance of a lithographic process, based on an in-house developed software package Norman-Debora. By first modeling the CD-dependency on various input variables (focus, dose, resist thickness, reticle CD,...), Norman predicts the CD-distribution based on assumed variation intervals for these input variables. The goal of this paper is to compare the predicted CD spread by Norman with the experimentally measured CD distributions focused on the poly layer of a quarter micron CMOS process.