A method includes providing a layout of an integrated circuit design, and generating a plurality of double patterning decompositions from the layout, with each of the plurality of double patterning decompositions including patterns separated to a first mask and a second mask of a double patterning mask set. A maximum shift between the first and the second masks is determined, wherein the maximum shift is a maximum expected mask shift in a manufacturing process for implementing the layout on a wafer. For each of the plurality of double patterning decompositions, a worst-case performance value is simulated using mask shifts within a range defined by the maximum shift.