A method for quantifying the effect of pupil function variations on a lithographic effect within a lithographic apparatus is disclosed. The method comprises: determining a discrete, two-dimensional sensitivity map in a pupil plane of the lithographic apparatus, wherein the lithographic effect is given by the inner product of said sensitivity map with a discrete, two-dimensional pupil function variation map of a radiation beam in the pupil plane. The pupil plane of a lithographic apparatus generally refers to the exit pupil of a projection system of the lithographic apparatus. Pupil function variations may comprise: relative phase variations within the pupil plane and/or relative intensity variations within the pupil plane.