1. A method comprising: generating, using a data processor, an average value of a parameter measured at various locations on a photo mask; and generating, using the data processor, information showing variations of the parameter across the photo mask relative to the average value of the parameter; wherein the information comprises data points, and each data point is determined based on a ratio between a measurement value and an average of a plurality of measurement values, and wherein the photo mask comprises a plurality of regions having repeating patterns, and the average of a plurality of measurement values comprises an average of values of measurements made at corresponding locations in the plurality of regions.