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Auctions An auction is a mechanism for trading items by means of bidding. Dates back to 500 BC where Babylonians auctioned off women as wives. (Herodotus) Prettiest girls are sold to the highest bidders, and the least attractive are offered w/ dowries funded by the former profits. Position of Emperor of Rome was auctioned off in 193 ad Can have the bidders trying to buy an item: Christies, ebay, snapnames. Can have the bidders trying to sell an item : Procurement, priceline.com Recently auctions were brought into both everyday use with the Internet (ebay and Google) and governmental arena with the spectrum auctions, bonds and environmental markets.

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Rules to Auctions First-Price Sealed-Bid Auction: Everyone writes down a bid in secret. The person with the highest bid wins the object and pays what he bids. Second-Price Sealed-Bid (Vickrey) Auction: Everyone writes down a bid in secret. The person with the highest bid wins the object and pays the second highest bid. (used for stamps and by Goethe) English Auction: The auctioneer starts at a reserve price and increases the price until only one bidder is left. Dutch Auction:The auctioneer starts at a high price and decreases the price until a bidder accepts the price. All Pay Auction: Everyone writes down a bid in secret. The person with the highest bid wins. Everyone pays.

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Tokyo Fish Auction Japan

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Dutch Auction (in Tokyo)

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Two types of Settings: Common and Private Examples of Common Value Auctions: Spectrum. Oil Drilling Book Example. Examples of Private Value Auctions: Consumption items. Memorabilia

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I am auctioning off the right shoe of the star of the Exeter City Football team. Al has value £30. Bob has value £40. Chris has value £55. What is revenue in the following situations? 2 nd -price sealed bid. 1 st price sealed bid. Each bids 2/3 of his value. English, Each bids up to his value Dutch, each bids 2/3 of his value. Private Value Auctions: Example:

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Strategies with Private values: English Auction The English: stay in the auction until either you win or the bid goes higher than your value. If not one either makes one lose when it is worthwhile to win or win when it is worthwhile to lose. The key to understanding this is to understand that staying in does not affect the price one pays if they win only whether one wins (it does affect others prices). It is best to do this independent of what others do.

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Strategies with Private Values: 2nd Price Auctions 2nd price similar logic to English auction. It is optimal to bid ones value. Ones bid does not affect the price one pays only whether or not one pays. Raising ones bid will cause one to win when it is not worthwhile. Your value is 5, the other team bids 6. If you bid 7, you will lose: 5- 6=-1. Lowering ones bid will cause one to lose when it was worthwhile to win. Your value is 5, the other team bids 4. If you bid 3, you wont win and could have won 5-4=1. Again it is best to do this independent of what others do.

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Strategies with Private Values: First Price Strategies in the first-price should shade bid below your value This is because ones bid affects ones price. Bidding your value will earn zero surplus. Shading ones bid lowers the probability of winning, but increases the surplus gained when winning. There is a natural trade-off between probability of winning and profit if one wins. If bid is b, value is v, expected profit is Probwin(b)*(v-b) Derivative of this w.r.t. b yields Probwin(b)*(v-b)-Probwin(b)=0 First term is marginal benefit of prob of winning. Second term is marginal cost of the profit if one wins.

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First Price Auction: Private values Example: Say the probability of winning Probwin(b)=2*b. What should you bid if you value is v? Algebraically, you should max 2b*(v-b). Note this is the equilibrium for 2 bidders, v uniform on [0,1]. Graphically, draw the Probwin vs. bid graph. Draw v on the x-axis. Expected profit is the box between b and v on the x- axis up to Probwin(b).

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First-Price Auction Profit with b1 is C+E, with b2 is D+E. Lowering bid gains A+B+D loses A+B+C Probwin b v b1b2 A B C DE

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All-pay auction May seem like a strange auction to run/study but… It is used in charity auctions and from the lab one can see why. (Losers dont complain so much.) Also a variant is played on Internet (avoid gambling laws). Extremely useful modelling tool. Patent Races/Technology Contests Political Campaigns. Procurement contests – Architecture, Next Generation Fighter Jet. Sports contests. (Think of Chelsea, Man U, Arsenal all buying the best players.)

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Strategies with Private Values: All Pay In the all-pay auction, you should again shade bid below your value. If bid is b, value is v, expected profit is Probwin(b)v-b. The natural trade-off is now between probability of winning and cost of bidding. This cost is incurred whether you win or not. Derivative of expected profits w.r.t. b yields Probwin(b)v-1=0 First term is marginal benefit of higher prob of winning. Second term is marginal cost of increased bid. It only makes sense to incur a high cost if the probability of winning is fairly high. For low values, bids are shaded much more than with first-price auctions.

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All-Pay Auction. If Probwin(b)=Sqrt(b), what is your choice of b. You should Max Probwin(b)*v-b. Note this is actually the equilibrium for 2 bidders, v uniform on [0,2].

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All-Pay Auction Profit with b1 is C+E-(F+G), with b2 is D+E-(A1+F). Lowering bid gains A2+B+D+G loses A1+A2+B+C Note Probwin(b) normally is not linear in all pay auctions. Probwin b v b1b2 A1 B C DE A2 F G 1

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Strategies with uniform values. Values are drawn from 0 to £10 with an equal chance of each amount. N is the number of bidders. 1 st -price the equilibrium bid (N-1)*v/N (that is if v=£5.50 and N=2, bid £2.75. Dutch auction is the same as the 1 st -price. 2 nd -price, optimal to bid value. English optimal to bid up to value. All-pay auction, should bid (N-1) * (v/10) N * 10/N (looks complicated but only we can see for low values shade bid more than for high values).

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Private values: Equilibrium Bid Functions 2 nd Price Bid Value 1 st -price All-Pay

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Classroom Experiments We ran 1 st price, 2 nd price, all pay one-shot. Good news: everyone copied their numbers correctly. Only two submitted two sets of bids. 124 students submitted bids. In 2 nd price 40 submitted optimal strategy, 16 too high, rest too low.

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Other classroom experiments In the past, we ran a number of designs in the lab including: First-Price with two buyers (random partners) Second-Price with two buyers (random partners) The buyers surplus is their combined profit. The sellers surplus is the sale revenue. Total surplus is buyers + sellers. Efficiency is 100*total surplus/potential surplus.

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Classroom Experiment Results First Price is best for the seller. (w/o all-pay) Second-Price is best for the buyers. First-Price is best for efficiency.

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Revenue Equivalence: 4 designs For private values, there is revenue equivalence among all 4 designs: 1 st /2 nd price, all-pay, English. Not only that but all auctions are fully efficient – the buyer who values the object the most winds up buying it.. Implies buyers surpluses are the same. If a seller wants to maximize revenue, he can simply use an appropriate minimum bid in any of the designs. Problems happen if: asymmetry, risk aversion, common values, seller info.

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Revenue Equivalence: General Two mechanisms that (1) yield the same allocation of the object. (2) have the same surplus for the bidder with the lowest valuation Have the following properties (1) The sellers expected revenue is the same. (2) A bidders expected surplus is the same. (3) Given a bidders value, his expected surplus is the same. Assumptions needed are risk neutrality, private and independent values. (Not Symmetry!)

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