Disclosed is a method of determining a process window within a process space comprising obtaining contour data relating to features to be provided to a substrate across a plurality of layers, for each of a plurality of process conditions associated with providing the features across said plurality of layers and failure mode data describing constraints on the contour data across the plurality of layers. The failure mode data is applied to the contour data to determine a failure count for each process condition; and the process window is determined by associating each process condition to its corresponding failure count. Also disclosed is a method of determining an actuation constrained subspace of the process window based on actuation constraints imposed by the plurality of actuators.