Preston McAfee is an unusual economist in that he has passed a market test: Yahoo is willing to pay him for his advice.
His latest advice
is on the design of auctions for graphical displays on web pages. Yahoo
runs billions of these automated auctions every day, through their
Right Media Exchange (RMX). Advertisers can bid for certain kinds of customers, and can pay for impressions, clicks, or sales.
To choose the highest-valued bidder, Yahoo develops predictors of how
many clicks and sales result from each impression. For example, if one
click occurs for every ten impressions, an advertiser would have to bid
more than 10 times as high for a click as for an impression in order to
win the auction.
Yahoo was very proud of its predictors, but was puzzled that they
systematically over-predicted the actual number of clicks or sales after
the auctions closed. A well-trained economist would recognize this as
an example of the winners' curse:
In a standard auction context, the winner’s curse states
that the bidder who over-estimates the value of an item is more likely
to win the bidding, and thus that the winner will typically be a bidder
who over-estimated the value of the item, even if every bidder estimates
in an unbiased fashion. The winner’s curse arises because the auction
selects in a biased manner, favoring high estimates. In the advertising
setting, however, it is not the bidders who are over-estimating the
value. Instead, the auction will tend to favor the bidder whose click
probability is overestimated, even if the click probability was
estimated in an unbiased fashion.
As with the winner's curse, there is a simple fix--bid as if your
estimate is the highest among all the bidders. This requires shading
your predictors downwards, based on the variance of the prediction.
The paper also details two other features that Yahoo uses to make their
auctions more efficient: randomized bidding, used when a less-informed
bidder bids against a more-informed one; and sometimes the auction is
not won by the highest bidder, due to the value of learning. When a
bidder has a novel or unusual use for the display, sometimes Yahoo lets
the novel user win so that Yahoo can learn more about the value of the
use. If Yahoo learns that the novel use of the display is more valuable
than they thought, then they can earn enough in the future to more than
compensate them for giving up some revenue earlier on.