Abstract: This invention provides a method for infrared and near-infrared estimations of properties of materials with improved model stability against random and pseudo-random effects and improved assessment of the appropriateness of the model for an unknown sample. Statistical tests are defined for quantifying the model's stability against random effects (B-statistic) and the probability of substantial interpolation error (I-statistic). The method of this invention involves building many different infrared models for each property of interest, either randomly or by other strategies. The models with the lowest calibration errors for their respective calibration sets whose errors are least correlated to one another are averaged to create a composite model. Such composite models have improved stability and accuracy. The consistency with which the individual models (making up the composite model) estimate the same property value for an unknown sample is a measure of the improbability of substantial interpolation error.