The combining classifier: to train or not to train?

When more than a single classifier has been trained for the same recognition problem the question arises how this set of classifiers may be combined into a final decision rule. Several fixed combining rules are used that depend on the output values of the base classifiers only. They are almost always suboptimal. Usually, however, training sets are available. They may be used to calibrate the base classifier outputs, as well as to build a trained combining classifier using these outputs as inputs. It depends on various circumstances whether this is useful, in particular whether the training set is used for the base classifiers as well and whether they are overtrained. We present an intuitive discussion on the use of trained combiners, relating the question of the choice of the combining classifier to a similar choice in the area of dissimilarity based pattern recognition. Some simple examples are used to illustrate the discussion.