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Strategic Behavior and Combinatorial Betting in Prediction

Dr. Yiling Chen
Yahoo! Research

Wednesday, March 5, 2008
3:30-5:000pm, W401 PBB

Abstract

Situated in a de facto standard market maker mechanism, logarithmic market scoring rules, we study prediction markets from both economic and computational perspectives. From the economic perspective, we investigate the equilibrium behavior of informed traders in prediction markets. We examine what information structures lead to truthful play by traders, meaning that traders reveal all of their information honestly as soon as they are able. We show that when signals of traders are independent conditional on the state of the world, truthful betting is a Perfect Bayesian Equilibrium (PBE). However, when signals are conditionally dependent, there exist joint probability distributions on signals such that at a PBE traders have an incentive to bet against their own information - strategically misleading other traders in order to later profit by correcting their errors. From the computational perspective, we design expressive betting languages for combinatorial prediction markets and examine the computational problem of pricing such markets. In our combinatorial markets, traders are allowed to submit bets of the form "horse A finishes in position 1, 2, or 5", "horse C beats horse D", "Hillary Clinton wins Ohio and Florida", or "Duke wins a third round game in the NCAA basketball tournament". Pricing such markets are computationally intractable except for the single-elimination tournament betting, in which case, we use a Bayesian network to facilitate price updates. This is the first example of a tractable market-maker driven combinatorial market.

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