1. A method of determining a property of a lithographic process, using a target structure that has been formed by the lithographic process on a substrate, the method comprising: forming an image of the target structure using an imaging system of a metrology apparatus that selects a certain portion of radiation diffracted by the target structure under certain illumination conditions; measuring the image of the target structure; identifying a region of interest in the measured image; and determining a value of at least one coefficient of a combination fit function using pixel values of the region of interest, wherein the value of the coefficient is indicative of the property of the lithographic process, and wherein the combination fit function has at least one fixed coefficient based on a characteristic or parameter (i) of the target structure, (ii) of the metrology apparatus, (iii) of the illumination of the target structure, the forming the image and/or the measuring the image, and/or (iv) of the lithographic process.