there are many other things you have to remember when interpreting prediction market data. first, the rules of the contract may be slightly different than your intuitive interpretation based on the market title. second, you need to consider how long to expiry and overall market volume- lots of times the shares will sit at otherwise strange odds because no one wants to park their money somewhere for so long, or because there isn't liquidity. third, you have to consider the nature of the market itself- any gambling outfit can tell you how what people want to happen will bias the wagers. and fourth, your are looking at a very restricted view- for instance, right now donald trump has a 13% chance of winning the electoral college on predictit. a study over a longer period of time would tell us more. i've followed predictit for us politics for about 5 years fairly closely, and there are certainly lots of spurious prices. but its also true that polls can be wild outliers. since both methodologies are at least in the same ballpark as far as accuracy, i think you need a bigger more rigorous dataset to make any kind of claim. i believe someone did with the iowa electronic markets back in the day, but i havent read it