Predictor variables definition

Model parameters can also be used to compute indices, which reflect the relative importance of the predictors in the definition of the model itself. In particular, two indices, the variable importance in projection (VIP) [43] and the selectivity ratio (SR) [44] are often used in the ambit of latent variable-based calibration. VIP is a measure of how much the individual variables contribute to the definition of both the X- and the Y-spaces in PLS-modelling. 

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