1. A method of analyzing spectral data for the selection of the best calibration model from a plurality of calibration models, relating spectra of a substance to a physical or a chemical parameter of the substance, over a predetermined range of the physical or chemical parameters, comprising the steps of: a) capturing spectral data of the substance with respective values of the physical or chemical parameter over the predetermined range; b) building a plurality of calibration models, using the captured spectral data from step a, in dependence upon the values of the physical or chemical parameter, said plurality of calibration models being based on said spectral data and physical or chemical parameter from step a and using statistical resampling methods to assess a predictive quality of the models; c) calculating tolerance intervals of the predicted results obtained using the models built in step b at reference level for each calibration model, and d) displaying the tolerance intervals at each reference level for each calibration model.