An improved method and apparatus are disclosed for processing spectral data to remove undesired variations in such data and to remove interfering information present in the data. The method land apparatus corrects multiplicative effects present in the spectral data. Additive and interferent contributions can be corrected as well. In one aspect of the method, coefficients for a selected appropriate model are applied to the input spectral data based on first and second reference spectra. The spectral data are then corrected based on the estimated coefficients at least as to multiplicative errors for producing a linear additive structure for use in calibration, validation and determination by linear multivariate analysis. The method and apparatus will improve the accuracy of spectral data structures derived from measurements Using spectroscopy, chromatography, thermal analysis, mechanical vibration and acoustic analysis, rheology, electrophoresis, image analysis and other analytical technologies producing data of similar multivariate nature.