A user may submit a search query to a search engine. The search engine may process the search query and generate a set of results. Each of the items searched by the search engine may have been pre-assigned to a category in a category tree. Previous interactions by other users with the items after similar queries may have been recorded. The search engine may identify categories based on the distribution of the interacted-with results among the categories. The category tree may be analyzed at different levels, based on the entropy observed at each level. A level with low entropy may be chosen, and categories at that level used to constrain the query.