1. A method for assessing the quality of a process model, the method comprising: receiving a mask layout; receiving a process model which models the effects of one or more semiconductor manufacturing processes on the mask layout; computing a gradient of the process model with respect to a process model parameter; computing a quality indicator at an evaluation point in the mask layout by convolving the gradient of the process model with the mask layout; and assessing the quality of the process model by comparing the quality indicator with a threshold, wherein if the quality indicator is below a first threshold, the process model is determined to be normally sensitive to the process model parameter, which indicates that the process model is of a high quality; wherein if the quality indicator is above a second threshold, the process model is determined to be overly sensitive to the process model parameter, which indicates that the process model is of a low quality, wherein if the quality indicator is above second threshold, the method further comprises refitting a statistical model to the process data to obtain a second process model that is of a better quality than the process model; and wherein the method is used to identify a high quality process model, which is subsequently used by an optical proximity correction (OPC) process.