The Overconfidence Index is defined as the ratio of the "home team vote" to the official bookmakers' chance of that country winning multiplied by 100.
For example, the English are four times as optimistic as the average bookmaker that England will win the World Cup. (..that's even more optimistic than Michael O'Malley at a Yankees game!)
If this grabs your interest too, here's the link to the study results, recapped in these headlines:
"Portuguese and U.S. Fans Most Overconfident about “Home Team’s” Performance Going into World Cup; Confidence in “Home Team” and Realism about Odds to Win the World Cup Varies Widely by Country, finds comScore Networks"