Disclosed is a process monitoring method, and an associated metrology apparatus. The method comprises: obtaining measured target response sequence data relating to a measurement response of a target formed on a substrate by a lithographic process to measurement radiation comprising multiple measurement profiles, wherein the measured target response sequence data describes a variation of the measurement response of the target in response to variations of the measurement profiles; obtaining reference target response sequence data relating to a measurement response of the target as designed to the measurement radiation, wherein the reference target response sequence data describes an optimal measurement response of the target in response to designed measurement profiles without un-designed variation; comparing the measured target response sequence data and the reference target response sequence data; and determining values for variations in stack parameters of the target from the measured target response sequence data based on the comparison.