1. A method of quantifying how well a photolithographic model simulates a photolithographic printing process, comprising: receiving a number of simulated features produced by a model that simulates how a test pattern will print on a wafer with a photolithographic process; receiving a description of the shapes of features printed on a wafer with the test pattern; comparing the shapes of the simulated features with the shapes of the printed features to quantify how the photolithographic model simulates the photolithographic printing process.