8. A method comprising: receiving, over a network from a client machine, a query; identifying a first plurality of categories responsive to the query based on prior user interactions after similar queries; identifying an entropy value based on a distribution of the prior user interactions over the first plurality of categories; and modifying the first plurality of categories based on the entropy value to identify a second plurality of categories; modifying the query to include the second plurality of categories as constraints; executing the query to identify search results, the query including the second plurality of categories as constraints; and communicating the search results, over the network, to the client machine.
9. The method of claim 8 , wherein the first plurality of categories includes categories within a tree of categories.