1. A computer-implemented method of calculating estimates of a joint posterior probability of class membership given combinations of attribute values of a pair of attributes, the calculating being performed on the basis of data representing a training set of a plurality of instances defined by attribute values for a plurality of attributes together with a class membership outcome, the method comprising: a) calculating first estimates of a posterior probability of class membership given attribute values of a first attribute of the pair; b) calculating second estimates of a posterior probability of class membership given attribute values of a second attribute of the pair; c) binning the first estimates into a plurality of first probability range bins; d) binning the second estimates into a plurality of second probability range bins; e) creating a mapping of instances of the training set mapped to combinations of one of each of the first and second pluralities of probability range bins; and f) on the basis of said mapping, calculating estimates of a joint posterior probability of class membership given membership in the combinations of first and second probability range bins.