A method for estimating a composition for a vacuum gas oil, VGO, composition, comprising: representing a FT-IR spectrum of a first VGO composition as a first weighted combination of FT-IR spectra from a database of FT-IR spectra; generating a model of composition for the first VGO composition based on the first weighted combination of FT-IR spectra; determining a first partial representation of the FT-IR spectrum of the first VGO composition as a second weighted combination of FT-IR spectra from the database of FT-IR spectra, the first partial representation being determined by partial least squares analysis, wherein the first partial representation corresponds to one or more spectral regions associated with a first compositional class, and wherein the first partial representation comprises a first leverage value and a first residual value; determining a second partial representation of the FT-IR spectrum of the first VGO composition as a third weighted combination of FT-IR spectra from the database of FT-IR spectra, the second partial representation being determined by partial least squares analysis, wherein the second partial representation corresponds to one or more spectral regions associated with a second compositional class, and wherein the second partial representation comprises a second leverage value and a second residual value; characterized in that the method further comprises the steps of: calculating a combined leverage value based on at least the first leverage value and the second leverage value; calculating a combined residual value based on at least the first residual value and the second residual value; identifying the first weighted combination of FT-IR spectra as having poor fit quality based on the combined leverage value being greater than one, the combined residual value being greater than one, or a combination thereof. representing a FT-IR spectrum of a first VGO composition as a first weighted combination of FT-IR spectra from a database of FT-IR spectra; generating a model of composition for the first VGO composition based on the first weighted combination of FT-IR spectra; determining a first partial representation of the FT-IR spectrum of the first VGO composition as a second weighted combination of FT-IR spectra from the database of FT-IR spectra, the first partial representation being determined by partial least squares analysis, wherein the first partial representation corresponds to one or more spectral regions associated with a first compositional class, and wherein the first partial representation comprises a first leverage value and a first residual value; determining a second partial representation of the FT-IR spectrum of the first VGO composition as a third weighted combination of FT-IR spectra from the database of FT-IR spectra, the second partial representation being determined by partial least squares analysis, wherein the second partial representation corresponds to one or more spectral regions associated with a second compositional class, and wherein the second partial representation comprises a second leverage value and a second residual value; characterized in that the method further comprises the steps of: calculating a combined leverage value based on at least the first leverage value and the second leverage value; calculating a combined residual value based on at least the first residual value and the second residual value; identifying the first weighted combination of FT-IR spectra as having poor fit quality based on the combined leverage value being greater than one, the combined residual value being greater than one, or a combination thereof.