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March 10, 2005

Crooked Numbers

The Only Constant Is Change

by James Click

A few weeks ago, when this column first got going, I did a bit of discussion about park factors, how they've changed, and whether they affect all types of pitchers equally.

Since then, there have been quite a few people talking about predictions for the upcoming season, and adjusting for players in new parks is an essential part of that process. Players like Shawn Green-–heading from Los Angeles to Arizona--are likely to see a boost in their raw numbers despite no actual gain, or even loss, in their performance. Understanding these changes is essential to running both fantasy teams and actual teams.

Adjusting for park factors going forward is more difficult than it might seem for one simple reason: Park factors are not as constant as they should be. Anyone who used the 2004 Player Forecast Manager and drafted the entire Expos team understands this problem very well. Nate Silver summed the issue up nicely in Baseball Prospectus 2005:

Previously, we had worked under the assumption that park factors would be the same in the upcoming season as they were in the previous season. This seems like a neutral enough premise, but it ignored the fact that park factors have fluke seasons, just as ballplayers do.

Certain parks are more difficult to predict than others. Tampa Bay, for example, has been a rock of stability since it opened in 1998. Its batter park factor was 104 in 1998, but stayed exactly the same at 100 from 1999-2003 before dipping to 96 last year. Likewise, Florida was also very steady, notching park factors of 96, 93, 96, 96, 97, 94, and 95 since 1998. But while the state of Florida may be very good at keeping its parks steady, several other locales are not. Here are the parks with the highest standard deviation in park factor since 1998, excluding cities in which a new park was built.

TEAM         1998  1999  2000  2001  2002  2003  2004  STDEV
Kansas City   104   101   104   110   117   113    95   8.2
Montreal       96   104   100   107   101   118    95   7.9
Colorado      119   129   131   122   121   112   120   6.8
Chicago (N)   103   107    90    96    98    99   106   6.4
Arizona       101    97   102   106   108   111   103   4.9
Texas         104   104   105   100   112   110   111   4.7
Anaheim       101   100   102   107    97    93    99   4.7

There are some reasons for the drastic changes in Kansas City and Montreal. The Royals moved their fences out prior to 2004 and Montreal added 20 games in San Juan; regardless, that’s a lot of room for error. Colorado has long held a dramatic edge on other parks, especially when mentally remembering park factors--yet in 2003 it was a mere 112 after that extreme 131 in 2000. Rather than the singular outlier to which we’ve all become accustomed, Coors Field was only the third-most advantageous park to hitters in 2003, following Kansas City and Montreal and barely edging out Arizona and Texas.

Looking at how each a previous season's park factor correlates to the current season over the past 15 years, we see a coefficient of correlation of .5522, strong enough to be significant, but low enough to raise concerns.