A method is presented for controlling a process to be applied to a patterned structure in a production run. Reference data is provided being representative of diffraction signatures corresponding to a group of different fields in a structure similar to the patterned structure in the production line, and of a control window for the process parameters corresponding to a signature representative of desired process results. The group of different fields is characterized by different process parameters used in the manufacture of these fields. The method utilizes an expert system trained to be responsive to input data representative of a diffraction signature to provide output data representative of corresponding effective parameters of the process. Optical measurements are applied to different sites on the patterned structure in the production line to obtain diffraction signatures of thereof and generate corresponding measured data. The expert system analyses the measured data to determine effective parameters of the process applied to the patterned structure in the production line. The effective process parameters can then be analyzed to determine deviation thereof from corresponding nominal values, to thereby enable the process control.