A method of estimating unknown property and/or composition data of a sample, comprising:
- (i) collecting respective spectra of n calibration samples, the spectra being quantified at f discrete frequencies (or wavelengths) and forming a matrix X of dimension f by n;
- (ii) producing a correction matrix U.sub.m of dimension f by m comprising m digitised correction spectra at said discrete frequencies f, said correction spectra simulating data arising from the measurement process itself;
- (iii) orthogonalising X with respect to U.sub.m to produce a corrected spectra matrix X.sub.c whose spectra are each orthogonal to all the spectra in U.sub.m ;
- (iv) collecting c property data, composition data, or property and, composition data for each of the n calibration samples to form a matrix Y of dimension n by c (c>1);
- (v) determining a predictive model correlating the elements of matrix Y to those of matrix X.sub.c ;
- (vi) measuring the spectrum of the sample under consideration at said f discrete frequencies to form a matrix of dimension f by 1;
- and
- (vii) estimating the unknown property and/or composition data of the sample under consideration from its measured spectrum using the predictive model.