A device manufacturing method is disclosed. A radiated spot is directed onto a target pattern formed on a substrate. The radiated spot is moved along the target pattern in a series of discrete steps, each discrete step corresponding to respective positions of the radiated spot on the target pattern. Measurement signals are generated that correspond to respective ones of the positions of the radiated spot on the target pattern. A single value is determined that is based on the measurement signals and that is representative of the property of the substrate.