I. Introduction
To combine Neural Networks (NNs) is more effective than trying to solve difficult problems by using a big size single NN. Not only an easier design and better accuracy can be obtained, but also a clearer understanding of how the resulting machine works is possible. In recent years, there have been several proposals to build ensembles of learning machines [1], [2]; Mixture of Experts (MoE) and Boosting are among the most relevant.