A method of determining the proportion of short-chain branching in an olefin copolymer process stream is disclosed. The short-chain branching may also be determined as a function of molecular weight in a sample having a range of molecular weights. In the method, at least two olefin copolymer training samples are provided. The respective samples have different, known proportions of short-chain branching. The infrared (e.g. FT-IR) absorbance spectra of the training samples in a wavenumber range are obtained. Calibration information is determined from the training samples by chemometrically correlating the differences in the infrared absorbance spectra of the training samples to the differences in the degree of short-chain branching in the training samples. This step generates calibration information that allows the degree of short-chain branching in a sample to be determined once its infrared absorbance spectrum is obtained. A method for determining the statistical error in the measurement of short-chain branching in an olefin polymer sample as a function of its molecular weight distribution is also disclosed.