Techniques for determining a value of a parameter of interest of a patterning process are described. One such technique involves obtaining a plurality of calibration data units from one or more targets in a metrology process. Each calibration data unit of at least two of the calibration data units represents detected radiation obtained using different respective polarization settings in the metrology process, each polarization setting defining a polarization property of incident radiation of the metrology process and of detected radiation of the metrology process. The calibration data units are used to obtain calibration information about the metrology process. A measurement data unit representing detected radiation scattered from a further target is obtained, the further target having a structure formed using the patterning process on the substrate or on a further substrate. A value of the parameter of interest is determined using the measurement data unit and the obtained calibration information.