1. A pattern inspection method comprising: acquiring an image of patterns formed on a substrate by scanning a substrate with a charged beam and detecting a charged particle generated from a surface of the substrate, to acquire an image of patterns formed on the substrate; measuring dimensional values of selected ones of the patterns based on the image; calculating a statistic of the dimensional values; determining whether a correction is needed on the basis of the calculated statistic; and if so correcting the dimensional value, and inspecting the selected patterns based on the corrected dimensional value, and if not, inspecting the selected patterns based on the measured dimensional value.