1. A method of characterizing a plurality of structures of interest on a semiconductor wafer, the method comprising: generating a plurality of models having varying combinations of floating and fixed critical parameters and corresponding simulated spectra, wherein each model is generated to determine one or more critical parameters for unknown structures based on spectra collected from such unknown structures; determining which one of the models best correlates with each critical parameter based on reference data that includes a plurality of known values for each of a plurality of critical parameters and corresponding known spectra, and for spectra obtained from an unknown structure using a metrology tool, selecting and using different ones of the models to determine different ones of the critical parameters of the unknown structure based on determining which one of the models best correlates with each critical parameter based on the reference data.