In one aspect the invention relates to an apparatus for analysing at least one measured optical profile of a machined workpiece, the apparatus comprising: an analysis module configured to: receive nominal optical profile data representing a desired profile of a workpiece as designed in a current design step; receive a plurality of measured optical profile data corresponding to a plurality of machined workpieces; for each measured optical profile data, determine a difference between the measured optical profile data and the nominal optical profile data; remove non-systematic machining errors from the determined difference, by determining an average of the plurality of determined differences; and output the determined average, corresponding to errors on the profile due to machining which are systematic machining errors, to a design module, in order to enable the design module to take into account the systematic machining errors in a further design step of the desired workpiece.