  Display advertising has traditionally been sold via guaranteed contracts -- a
guaranteed contract is a deal between a publisher and an advertiser to allocate
a certain number of impressions over a certain period, for a pre-specified
price per impression. However, as spot markets for display ads, such as the
RightMedia Exchange, have grown in prominence, the selection of advertisements
to show on a given page is increasingly being chosen based on price, using an
auction. As the number of participants in the exchange grows, the price of an
impressions becomes a signal of its value. This correlation between price and
value means that a seller implementing the contract through bidding should
offer the contract buyer a range of prices, and not just the cheapest
impressions necessary to fulfill its demand.
  Implementing a contract using a range of prices, is akin to creating a mutual
fund of advertising impressions, and requires {\em randomized bidding}. We
characterize what allocations can be implemented with randomized bidding,
namely those where the desired share obtained at each price is a non-increasing
function of price. In addition, we provide a full characterization of when a
set of campaigns are compatible and how to implement them with randomized
bidding strategies.

Multiagent Systems;Computer Science and Game Theory