1. A method for estimating yield of a wafer having a plurality of chips of a given design printed thereon, the method comprising the steps of: dividing the chip design into a plurality of rectangular cells, wherein each of the cells is chosen to be small enough such that focus and dose values across the cell can be assumed to be constant; determining a process window for each of the cells; measuring the focus and dose values on the wafer; using the measured focus and dose values to determine a Gaussian random component of the focus and dose values; representing the focus and dose values on the wafer as a sum of a systematic component of the focus and dose values and the Gaussian random component of the focus and dose values; and estimating wafer yield based on a number of the chips on the wafer for which at each point (x, y) the focus and dose values, as represented as the sum of the systematic component of the focus and dose values and the Gaussian random component of the focus and dose values, belong to a corresponding one of the process windows.