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February 10, 2005

Crooked Numbers

More Time in the Park

by James Click

I’ve been on a little bit of a park factor kick as of late.

Last week was spent checking to see if groundball pitchers were less affected by park factors than flyball pitchers are, a theory based on the assumption that park factors are based largely on outfield dimensions. This turned out not to be the case. Months before that was a little foray into park factors and baserunning attempt and success rates, checking to see if perhaps home teams got some of their inherent advantage from knowing how the ball bounces in their yard better than their visiting opponents do. Again, the theory did not pan out.

I’m back for more. At the heart of what I was trying to do last time was the idea that park dimensions are a central part of park factors. But because I was trying to take something absolute--the outfield dimensions--and map it to something relative--the park factor--via groundball/flyball numbers, it occurred to me that there would be problems with that kind of analysis when dealing with multiple seasons. Even single seasons can be messy when the Mariners do annoying things like open a new park in the middle of July.

As some parks change and new ones open, park factors change. The Oakland Coliseum used to have a bit of a reputation as a pitchers' park, but in the last few seasons it’s actually started to play as a slight hitters' park. There are many possible reasons for the change: the construction of Mt. Davis (the monstrosity that replaced the outfield bleachers) when the Raiders moved back to town, earthquakes, general changes to wind patterns, and--perhaps most probable--the construction of other parks in the league that changed how the Coliseum is perceived in relation to them.

The big problem in finding some truth is that so many new parks have opened in the last 15 years that park factors haven’t had any chance to steady themselves. Each year, a new park or two changes the mix. So if we were trying to determine if the construction of Al Davis’ monument to litigation had an effect on the Coliseum's park factor, we would first have to correct for all the other parks being built around the league during that time. Mapping any specific features to the park factor is made significantly more difficult because even those parks that don’t change have park factors that do.

This begs the questions: can we determine how the new parks are changing the overall league performance level? Are some of the offensive numbers we’ve been seeing as of late a result of a series of hitter-friendly ballparks being introduced around the league?