In a method of determining a structure parameter of a target pattern in a lithographic process, a series of calibration spectra are calculated from a reference pattern. Spectral analysis is performed on each calculated spectra, the spectral components and associated weighting being derived and stored in a library or used as the basis of an iterative search method. A spectrum is measured from the target pattern and spectral analysis of the measured spectrum is performed. The derived weighting factors of the principal components are compared with the weighting factors of the measured spectrum to determine the structure parameter.