A method of determining a structural parameter related to process-induced asymmetry, the method including: illuminating a structure, having an asymmetry property and a sub-structure susceptible to process-induced asymmetry, with radiation with a plurality of illumination conditions, at a first location of a substrate, determining a difference between measured asymmetry properties of the structure obtained with each of the plurality of illumination conditions, calculating a differential dependence of a difference between modeled asymmetry properties simulated for illumination by each of the plurality of illumination conditions on a structural parameter representing process-induced asymmetry of the sub-structure, and determining a value of the structural parameter using the determined difference and the calculated differential dependence.