  In the standard single-dimensional model of position auctions, bidders agree
on the relative values of the positions and each of them submits a single bid
that is interpreted in terms of these values. Motivated by current practice in
sponsored search we consider a situation where the auctioneer uses estimates of
the relative values, which may be imprecise, and show that under both complete
and incomplete information a non-truthful mechanism is able to support an
efficient outcome in equilibrium for a wider range of these estimates than the
VCG mechanism. We thus exhibit a property of the VCG mechanism that may help
explain the surprising rarity with which it is used even in settings with unit
demand, a relative lack of robustness to misspecification of the bidding
language. The result for complete information concerns the generalized
second-price mechanism and lends additional theoretical support to the use of
this mechanism in practice. Particularly interesting from a technical
perspective is the result for incomplete information, which is driven by a
surprising connection between equilibrium bids in the VCG mechanism and the
generalized first-price mechanism.
