1. A computer-implemented method comprising: obtaining search activity data of a plurality of search events, wherein each of the plurality of search events relates to a search of one of a plurality of index-type search engines based on a search query, a resulting set of search results, and a selection of a search result from the set of search results, wherein each of the plurality of index-type search engines comprises a proprietary index for generating search results; identifying a subset of search events from the plurality of search events, wherein each of the search events in the subset of search events relates to a referral to a particular search destination based on the selection of a search result from the set of search results for the search event; and storing, in computer storage, for each search event in the subset of search events, at least an indication of the search engine associated with the search event, an indication of the search query associated with the search event and an indication of a position of the selected search result from the set of search results associated with the search event.