1. A method for selecting useful channels in a non-invasive medical device comprising: embedding a discrete predictor problem into continuous space of a predictor preweighting; executing an optimization algorithm to optimize a preweighting vector, wherein the optimization algorithm determines the relative importance of the each predictor; constructing n different models, wherein a k th model comprises the k most important predictors; comparing the models using an information criterion to allow for automatic selection of a subset of the predictors; and deriving an analytical Jacobian of the partial least squares regression vector with respect to the predictor weighting to optimize the predictor weighting and select useful channels for use in the non-invasive medical device.