Data processing apparatus is provided for evaluating answers to respective query items considered to be represented by respective points within a region of feature space, and the apparatus comprises an input (10) which receives such a query item. The region is subdivided into subregions according to at least first and second subdivisions. A subregion identifying portion (20) identifies, for each such subdivision of the region, which subregion of the subdivision contains the point representing the received query item. A partial answer retrieval portion (30) has access when the apparatus is in use to a store (40) of precalculated partial answers for at least some the subregions of the subdivisions, and retrieves from the store the partial answers for the or each identified subregion that is present in the store. An answer calculation portion (50) calculates an answer to the received query item based on the retrieved partial answers, and an output (60) outputs the calculated answer.