Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving queries, and for each received query: receiving data indicating resources identified by a search operation as being responsive to the query, wherein the search operation scores each of the resources based, in part, on features of the resource and the query, selecting a subset of the resources, determining, from the subset of resources and for each of the features of the resources and the query, an impact measure that measures the impact of the feature on the ranking of the resources that belong to the subset, adjusting the search operation based on the respective impact measures, and initiating the search operation to re-score the resources in the subset of resources based, in part, on the adjustment and to rank the subset of resources according to a second order that is different from the first order.