17. An information retrieval system comprising: an input arranged to receive a query; an output arranged to issue the query to a plurality of sources; the input being arranged to receive a results list from each of the plurality of sources; memory storing a merging model in a data structure comprising a plurality of weighted event variables describing a complete results list presentation event, the merging model comprising a decision process that determines whether or not to merge results based on the relevance of the results, the decision process deciding to not merge results when a difference between a first measure of relevance of a first set of results and a second measure of relevance of a second set of results satisfies a threshold condition; a processor forming a complete results list from the received results lists using the merging model; and a user interface arranged to present the complete results list; wherein the processor is arranged to observe user behavior in response to the presented complete results list and to use the observed user behavior to update the merging model.
17. An information retrieval system comprising: an input arranged to receive a query; an output arranged to issue the query to a plurality of sources; the input being arranged to receive a results list from each of the plurality of sources; memory storing a merging model in a data structure comprising a plurality of weighted event variables describing a complete results list presentation event, the merging model comprising a decision process that determines whether or not to merge results based on the relevance of the results, the decision process deciding to not merge results when a difference between a first measure of relevance of a first set of results and a second measure of relevance of a second set of results satisfies a threshold condition; a processor forming a complete results list from the received results lists using the merging model; and a user interface arranged to present the complete results list; wherein the processor is arranged to observe user behavior in response to the presented complete results list and to use the observed user behavior to update the merging model. 18. An information retrieval system as claimed in claim 17 wherein the processor is arranged to observe both explicit and implicit feedback.