A method including: obtaining error information indicative of accuracy of positioning a pattern formed on a layer on a substrate relative to a target position, wherein the pattern has been formed by irradiating the layer with a radiation beam patterned by a patterning device; and producing modification information including a map of positional shifts across the patterning device so as to increase the accuracy of positioning the pattern formed using the patterning device modified according to the modification information, the modification information based on the error information, wherein the error information is independent of any other layer on the substrate.