An arbitrage opportunity

Nate Silver noted a discrepancy between the market prices for bets on who will win the presidential election at Intrade vs. other markets like the Iowa Electronic Market, with strange bidding patterns at Intrade. More discussion over at Computational Complexity. To help visualize it, here is a graph of the closing prices of PRESIDENT.DEM2008 at Intrade and PRES08_WTA at IEM. Update: CQpolitics: Trader Drove Up Price of McCain ‘Stock’ in Online Market It certainly looks as though there's been an unusually large difference in prices lately.