A method for determining an assay property of an unknown material, wherein said unknown material is crude oil, a gas oil feed to a catalytic cracking unit, or a waxy distillate feed to a lube extraction unit, and said assay property is any physical, chemical or performance property of the whole crude and any distillation cut thereof, or of the feed and any subfraction thereof, the method comprising: a) determining multivariate analytical data of said unknown material using multivariate analytical techniques, wherein the multivariate analytical techniques comprise spectroscopy, chromatography, or spectrometry, or their combinations; b) fitting said multivariate analytical data to a linear combination of known multivariate analytical data in a database to determine the coefficients of the linear combination, wherein said database includes multivariate analytical data of reference materials whose assay properties are known, and (c) determining said assay property of said unknown material from the coefficients and assay properties of said reference materials, characterized in that the method further comprises the steps of: eliminating signals from the multivariate analytical data that are not related to the molecular constituents of the material, wherein the step of eliminating signals is performed by orthogonalizing the multivariate analytical data of the reference and unknown materials to example data of the signals to be eliminated, and augmenting said multivariate analytical data with inspection data to form augmented data such that said augmented data of the unknown material is fit to a linear combination of multivariate analytical data augmented with inspection data of the known reference materials, wherein the inspection data comprises API or specific gravity and/or viscosity, and wherein said inspection data is in volumetrically blendable form, such that the data values for a mixture can be calculated as the sum of the products of the volumetric proportions of the components of the mixture times the component properties. a) determining multivariate analytical data of said unknown material using multivariate analytical techniques, wherein the multivariate analytical techniques comprise spectroscopy, chromatography, or spectrometry, or their combinations; b) fitting said multivariate analytical data to a linear combination of known multivariate analytical data in a database to determine the coefficients of the linear combination, wherein said database includes multivariate analytical data of reference materials whose assay properties are known, and (c) determining said assay property of said unknown material from the coefficients and assay properties of said reference materials, characterized in that the method further comprises the steps of: eliminating signals from the multivariate analytical data that are not related to the molecular constituents of the material, wherein the step of eliminating signals is performed by orthogonalizing the multivariate analytical data of the reference and unknown materials to example data of the signals to be eliminated, and augmenting said multivariate analytical data with inspection data to form augmented data such that said augmented data of the unknown material is fit to a linear combination of multivariate analytical data augmented with inspection data of the known reference materials, wherein the inspection data comprises API or specific gravity and/or viscosity, and wherein said inspection data is in volumetrically blendable form, such that the data values for a mixture can be calculated as the sum of the products of the volumetric proportions of the components of the mixture times the component properties.