16. A computer-implemented method as claimed in claim 15 which further comprises training the model using historical {x, y, a} triplet values.
15. A computer-implemented method as claimed in claim 14 which comprises using a machine learning system comprising a stored probabilistic model of the form p(y|x,a) to make the prediction by applying the model to give the probability p of observing y after selecting a when x is observed, where the symbol x represents inputs to the prediction engine, the symbol y represents outputs from the model being predicted relevance information, and the symbol a represents a list of documents.