1. A method of calibrating a model of a lithographic process, the method comprising the steps of: (a) providing a lithographic process model comprising a function of combinations of image parameters and coefficients of said combinations of said image parameters; (b) providing a calibration pattern having measurement sites for determining model parameters, said model parameters comprising at least one of said image parameters; (c) determining for at least one of said model parameters a set of bin values of said at least one of said model parameters, said set of bin values defining a parameter space between a minimum parameter value and a maximum parameter value; (d) transferring said calibration pattern in accordance with the lithographic process to form a printed image; (e) obtaining measured values of said at least one of said model parameters at said measurement sites in said printed image; (f) determining values of said coefficients so that a simulated image of said calibration pattern produced by using said lithographic process model provides simulated values of said at least one of said model parameters that match said measured values within a predetermined tolerance; (g) classifying said simulated values of said at least one of said model parameters in accordance with said set of bin values; (h) evaluating whether said parameter space is adequately populated by said classified simulated values; and (i) if said parameter space is not adequately populated, then modifying said calibration pattern and repeating the steps (c) through (i) using said modified calibration pattern until said parameter space is adequately populated.