A system and method for determining parameters such as critical dimension of a patterned structure and best focus condition of a lithographic apparatus, based on measurement of the intensity of a non-zero order of light diffracted from an experimental structure. The experimental structure includes a first array of lines and partially filled spaces having a period longer than the wavelength of the diffracted light. The experimental structure also includes a second array of lines and spaces, where the second array of lines and spaces comprise the partially filled spaces.