1. A method comprising: identifying, by a computing device, a referral to a website, the referral made in response to selection of a referral resource at a search results page of a search engine; identifying, by the computing device, a search query submitted to the search engine to obtain the search results page; determining, by the computing device, reference activity data comprising data indicative of a plurality of search queries submitted to the search engine from a plurality of computing devices, the plurality of search queries comprising search queries that are similar to or the same as the search query; generating, by the computing device, statistical data based at least in part on the reference activity data, the statistical data comprising one or more of a number of the plurality of search queries, a frequency of the plurality of search queries, and variation in usage of the plurality of search queries; and determining, by the computing device, whether the referral to the website represents click fraud based at least in part on a comparison of the search query to the statistical data.