Abstract: Methods are disclosed for quantifying physical properties of gaseous, liquid or solid samples. The near-infrared absorbance spectra of a representative field of calibration samples are measured and recorded using a spectrophotometer. The absorbance spectra of the calibration samples are evaluated by a row-reduction algorithm to determine which wavelengths in the near-infrared spectrum, and associated weighting constants, are statistically correlated to the physical property being quantified. The near-infrared absorbance of actual samples is then measured at each of the correlated wavelengths, and then corrected by the corresponding weighting constants. A reference value for the physical property being quantified is then computed from the corrected measure of the absorbance of the sample at each of the correlated wavelengths.