1. A method of generating targeting parameters of a media buy plan for advertisements to be displayed in conjunction with presenting web pages, based on a history of search events, comprising: receiving key phrases relative to a subject of the advertisements to be displayed; providing the received key phrases as proposed key phrases to determine, from search events indicative of historical data of uses of a search service, a first subportion of search events for queries of the search service with the proposed key phrases and a second subportion of search events for queries of the search service not with the proposed key phrases; applying classification processing to determine potential targeting parameters associated with the first subportion and with the second subportion to identify potential targeting parameters that, statistically, contribute to membership in the first sub-population and in the second sub-population, respectively; and associating statistics with the potential targeting parameters, based on the historical data, indicative of factors usable to determine whether to use the potential targeting parameters as actual targeting parameters of the media buy plan.