Park Indices: Wrigley Field, US Cellular
If you haven't picked up the 2011 Bill James Handbook, please do so. There's some great information in there regarding hitter and pitcher projections, plus/minus (defense), career data and park indices.
Here's the macro level view for park indices (home runs)...
A park index of exactly 100 is a neutral park -- it has no effect on this particular stat. An index above 100 favors the statistic that it is easier to hit home runs in said ballpark. The opposite is also true - if the index is below 100, it is more difficult to hit home runs in said ballpark.
In 2010, Wrigley Field had a park index of 110. Meaning it's 10% easier to hit HRs in Wrigley than in a neutral park. The Cubs hit 74 home runs in home games while their opponents hit 87. Miller Park had a park index of 123, Busch Stadium had an index of 77, and Great American Ballpark had an index of 114.
U.S. Cellular Field had a park index of 157. WOWZA! It was 57% easier to hit a HR in
Comiskey Park The Cell than in a neutral park. The White Sox hit 111 home runs at home while their opponents hit 79. The index of 157 makes the Cell the easiest place to hit home runs in 2010. This was more than Coors Field (144) and Yankee Stadium (143). The park index for Target Field (where those pesky Twins play) was 65. Yup - 35% more difficult to hit a home run in Target Field than in any park in the majors.