2. The method of claim 1 further comprising, at the search engine web server: calculating a numerator as a ratio of an estimated number of times the clickthrough events will occur to an estimated number of times the clickthrough events could occur within a specified time period; calculating a denominator representing the standard click-through rate for listings of the average ranking, over the same specified period of time, of the identified search listing corresponding to the bid; and calculating the probability as the ratio of the numerator to the denominator.
1. A method of operating a search system, the method comprising: maintaining a database of search listings searchable by a search engine, each search listing being associated with a search term, a respective web site promoter and one or more bid element amounts, the one or more bid element amounts corresponding to money amounts chargeable to the respective web site promoter when a search listing is referred by the search engine; at a search engine web server in data communication with the database, receiving a search request from the searcher; at the search engine web server, identifying search listings in the database which generate a match with the search request; at the search engine web server providing search listings from the database to a searcher in conjunction with a page of information, including providing with the provided search listings a search result list which includes the identified search result listings, including ordering the identified search listings into the search result list in accordance with values of respective bid element amounts for the identified search listings, wherein the values of the respective bid amounts comprise: (a) a value of a bid element amount for impression; (b) a value of a bid element amount for clickthrough, calculated based on the bid element amount and probability of a clickthrough event, the probability being calculated as a ratio of an estimated number of times the clickthrough events will occur to an estimated number of times the clickthrough events could occur within a specified time period; and (c) a value of a bid element amount for actions; subsequently, receiving at the search engine web server, none, one or more of the following: a retrieval request from the searcher to retrieve information associated with a selected search listing in the provided search listings, and one or more notifications that the searcher has performed one or more respective actions in connection with the selected search listing; at an account management server in data communication with the search engine web server, debiting an account of the web site promoter associated with the selected search listing as follows: by a bid element amount for impression when the search listing is displayed to the searcher, by a bid element amount for a clickthrough, when a clickthrough request was received as a notification, and by a bid element amount for one or more actions when notifications were received for said one or more actions.