  Some online advertising offers pay only when an ad elicits a response.
Randomness and uncertainty about response rates make showing those ads a risky
investment for online publishers. Like financial investors, publishers can use
portfolio allocation over multiple advertising offers to pursue revenue while
controlling risk. Allocations over multiple offers do not have a distinct
winner and runner-up, so the usual second-price mechanism does not apply. This
paper develops a pricing mechanism for portfolio allocations. The mechanism is
efficient, truthful, and rewards offers that reduce risk.
