1. A system comprising: at least one processor; and a query expansion classifier module, executable by the at least one processor, configured to: receive a query expansion source, a query expansion candidate, and feature data for the query expansion source and the query expansion candidate, the feature data comprising information for a plurality of features, the plurality of features comprising a measure of price divergence for the query expansion source and the query expansion candidate, the measure of price divergence being a measure of similarity between a distribution of prices of item listings returned as results for a search on the query expansion source and a distribution of prices of item listings returned as results for a search on the query expansion candidate, and the measure of price divergence comprising a Student's t-statistic; determine whether the query expansion candidate qualifies as an expansion of the query expansion source based on an analysis of the information for the plurality of features; and assign the query expansion candidate as an expanded query of the query expansion source in a query expansion dictionary in response to a determination that the query expansion candidate qualifies as an expansion of the query expansion source.
5. The system of claim 1 , wherein the plurality of features comprises a measure of category divergence for the query expansion source and the query expansion candidate, the measure of category divergence being a measure of similarity between a distribution of item categories of item listings returned as results for a search on the query expansion source and a distribution of item categories of item listings returned as results for a search on the query expansion candidate.