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A sponsored search auction (SSA), also known as a keyword auction, is an indispensable part of the business model of modern web hosts. It refers to results from a search engine that are not output by the main search algorithm, but rather clearly separate advertisements paid for by third parties. These advertisements are typically related to the terms for which the user searched. They come in the form of a link and some information, with the hope that a search user will click on the link and buy something from the advertiser. In sponsored search auctions, there are typically some fixed number of slots for advertisements and more advertisers that want these slots than there are slots. The advertisers have different valuations for these slots and slots are a scarce resource, so an auction is done to determine how the slots will be assigned. ==History== Prior to 1998, many advertisements were charged by impression, as it was the easiest metric to calculate. In 1998, GoTo.com, Inc debuted a pay-per-click charging system, with pricing and slot placement determined by an auction. GoTo used a first price auction, where bidders were placed according to their bids and charged their bids when they won. GoTo faced bidders who were constantly changing their bid in response to new information and changing information from other bidders. Currently, charging per action is a common pricing scheme in affiliate networks, such as the Amazon Associates Program. In 2002, Google AdWords began using a second price auction to sell the single advertisement slot. Shortly thereafter, pages had multiple advertisements slots, which were allocated and sold via generalized second-price auction (GSP) auction, the natural generalization of a second price, single item, multi bidder auction.〔Hal Varian, Christopher Harris. The VCG Auction in Theory and Practice, In The American Economic Review, Volume 104, Issue 5, pages 442-452. Elsevier B.V., 2014.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Sponsored search auction」の詳細全文を読む スポンサード リンク
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