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CBP's Park Effects: Like Night and Day?

Something strange seems to be happening at Citizens Bank Park (CBP) lately. Even as the park's dimensions have generated local and national bandbox hysteria, a handful of recent games have left the scoreboard operator with very little to do. On July 9, the Phils beat the Nationals 1-0. On July 21, they lost to the Dodgers by the same score. Those were the first two 1-0 games ever at CBP. Two days later, the Phils beat the Padres by the score of 2-0.

That's three games over two weeks with a total of four runs scored. At the ballpark John Smoltz has claimed is such a "joke" that it isn't even deserving of the label "baseball field." The park David Wells proclaimed plays like the Little League World Series. What's going on?

Well, so far this year, CBP has split personality disorder. During the day, CBP plays like a slightly hitter-friendly ballpark that would hardly be worth noticing. The three low-scoring games in the last couple of weeks started at 1:20, 1:05, and 3:15 respectively. At night, however, the park changes. Every pitcher?s fear comes true, and every hitter?s greatest dream is fulfilled: When the lights come on, CBP morphs into Coors Field at sea-level.

To illustrate this difference, we can compare CBP?s daytime and nighttime park factors. In its simplest form, park factor measures the increase or decrease in offense at a park by comparing the runs scored in a team?s games at home and the runs scored in a team?s games on the road. Park factors can do the same for other game events as well, such as hits, home runs, etc.

General park factors are easy to find for a stadium. ESPN lists them, although on a webpage that is frustratingly unpredictable in its accuracy. Baseball Prospectus and Baseball Reference do so as well. Each uses a slightly different formula, but for this study, I?ll use the simple formula described at the top of ESPN?s park factor page: ((homeRS + homeRA)/(homeG)) / ((roadRS + roadRA)/(roadG)). Instead of just looking at the blanket measure, though, I?ll compare Phils home night games to night games away and home day games to day games away.

Here?s the resulting chart with the 2005 CBP park factors, broken down by day games and night games, for runs, home runs, hits, walks, and strikeouts (through Monday, July 25 - the team?s first 100 games).

2005            R       HR      H       BB      SO
Day             1.067   1.179   1.120   0.980   0.966
Night           1.543   1.662   1.218   1.022   0.939
All             1.353   1.515   1.180   1.013   0.949

Translating the numbers into simple concepts, in day games at CBP, teams score 6.7% more runs and slug 17.9% more home runs compared to day games on the road. The stadium also increases hits and slightly decreases walks and strikeouts. Overall, during the day, the stadium inflates offense, but not drastically so. Phillies? pitchers? ERAs demonstrate this: they have a very good 3.62 home day ERA and an even better 3.38 road day ERA.

At night, though, CBP lives up to its reputation as the extreme park pitchers and fans bemoan: scoring goes up an incredible 54.3% and home runs by an even more astounding 66.2% (meaning nearly two-thirds more home runs are hit at night games at CBP than in night games elsewhere). It is this unbelievable night offense that has made CBP into an overall hitters? haven this year. This night park effect has also been responsible for a drastic split in the Phillies? team ERA: they have a Rockies-esque 5.75 home night ERA and a 4.18 away night ERA (which is just about a league average ERA).

The numbers for CBP so far this year come from a very small sample size - 20 day games and 35 night games - so it?s worth comparing them to last year?s park effects:

2004            R       HR      H       BB      SO
Day             1.109   1.368   1.030   0.986   1.043
Night           0.977   1.027   0.955   1.020   0.994
All             1.024   1.134   0.981   1.009   1.010

The park?s many critics might not believe it, but last season CBP inflated offense by a ho-hum 2.4%. Nor was the day/night split especially noteworthy: CBP inflated offense by 10.9% during day games and actually deflated offense by 2.3% at night. Hits, walks, and strikeouts were generally neutral both during the day and at night. The only difference was that home runs were inflated during the 25 day games much more than during the 56 night games. Other than this difference, the park played pretty much the same during the night and day.

Looking at all of these numbers together, it appears that this year?s day numbers would actually fit quite nicely among last year?s numbers. On the other hand, the night numbers from this year look like a drastic outlier compared to last year. With over 100 games (81 from last year and the 20 day games from this year) showing CBP as a slightly hitter-friendly park but not at all drastically so, it is reasonable to speculate that the extreme increase in offense for the 35 night games so far this year is merely a statistical oddity rather than an accurate depiction of CBP?s true park effect.

If this hypothesis proves to be true, the hysteria over CBP being Coors Field East is misplaced, and all we have on our hands is a sample-size fluke showing 35 games of inflated offense. We won?t have our answer about the stadium?s true effects until more games are played at CBP. However, refining our analysis of CBP?s effects based on when games are played indicates there?s a lot more to understanding the stadium than just jumping on the bandbox bandwagon.