Finally, we looked at online trading. Intrade, the online prediction outfit used for non-sports-related events has offered contracts for the major candidates throughout the campaign season. These contracts trade at US$0.10 per point, in a range of 1-100 points. Because prices are limited to that range, a price of 50 points for a candidate's contract indicates that the market is estimating a 50% probability of that candidate winning the race.

Interestingly, if we examine the trends in daily closing prices for the candidates' respective contracts, we find them mapping reasonably well onto the broad measures of popularity that we found when looking at traffic to their respective campaign Web sites. Obama's contract was comfortably above 60 points for much of the summer. It then fell below 50 points (50% probability) for the first time in the week following the Republican convention. During that same week, McCain's contract rose above 50 points for the first time in the campaign. Then, just as quickly as they had converged, the trends lines separated again, with the Obama contract rising, and the McCain contract falling, from mid-September on.

These developments obviously correspond quite well to what we noted in most of the site traffic data -- reflecting both Obama's summer-long advantage over McCain, as well as the temporary, convention-related convergence in their popularity in early September (the "pinch").

Moreover, the online prediction market seems to reflect the same more-recent trend noted with Quantcast's and Alexa's traffic numbers: a sharp increase, starting from about the middle of September, in Obama's typical advantage over McCain, with the Obama contract rising over 70 points for the first time in early October, just as McCain's contract was falling below 30 points for the first time since mid-July. Following the third Presidential debate (Oct. 15) Obama's Intrade contract was trading at a new high of over 80 points, and McCain's at a new low of below 20.

Rigging the system?