Sunday, January 6, 2008

The Fenway Park Effect - Introduction

Ever wonder how Dustin Pedroia managed to hit .317 as a rookie? Perhaps you're left scratching your head over how Drew's home run total was cut in half in 2007? Maybe you want to know if Fenway Park really does cause errors, as Renteria claimed in 2005? Well thanks to the Bill James Handbook, we have some answers.

Now park effect doesn't explain or account for everything. Obviously, Pedroia has to be able to hit to get a .317 batting average, regardless of the park he plays in. Park effects mostly help us to understand some trends in pitching and hitting performance. It's especially useful when trying to understand how players' performances may be affected by switching teams. At the same time, it works to create more answers. Such as how is it that the Red Sox lead the AL in ERA, while also pitching in the league's most hitting friendly park?

First of all, I'll talk about Fenway Park's effect on producing runs. A park rating of 100 is neutral, regardless of the category. In the category of producing runs, Fenway Park received a rating of 118 in 2007. That rating lead the majors, but what exactly does it mean? A rating of 118 means that it was 18% easier to score a run in Fenway Park, than it was to score a run in a neutral run scoring park.

So what does such a rating tell us? The first conclusion I'd come to is that the Red Sox offense is probably a bit overrated. If it is 18% easier to score a run in Fenway Park, than it is a neutral run scoring park, Red Sox offensive numbers are probably inflated. This could help to explain how the Red Sox have ranked among baseball's top five offenses in five out of the last six years.

But that's only half the story. If it's so much easier to score runs in Fenway Park, then what's that mean for the Red Sox pitchers? No doubt, if it's easier to score runs in Fenway Park, it's also much harder to prevent runs. But how much more difficult is it for Red Sox pitchers to prevent runs?

In a perfect world, the average American League park would be neutral. And since the Red Sox play half their games at Fenway Park, it would be 9% harder for Red Sox pitchers to prevent runs, assuming they make half their starts at home and half on the road. And in this case, Beckett's already sterling 2007 ERA of 3.27 would look more like 2.97 had Fenway Park been a hitter neutral park.

But we don't live in a perfect world. In reality, Beckett pitched almost 20 more innings at home than he did on the road last year. And the average American League park wasn't neutral. The average American League park last year was somewhere between Comerica Park and Kauffman Stadium, and was around 5% easier to score a run in than a neutral park.

So I'll account for the fact that Beckett pitched 54.8% of his innings at home, and the fact that the average AL park had a run scoring rating of 105 or +5%. Now, if Beckett pitched in a hitter neutral park, his ERA would have looked more like 3.11. So was he robbed of a Cy Young Award?

Not so fast, the second most run scoring friendly park in the American League was Jacob's Field, where C.C. Sabathia pitched his innings when he was at home. If you adjust both of their home innings to the neutral American League park, their numbers would look more similar, but only slightly.

And my calculations above ignored the fact that Beckett pitched many of his starts in NL Parks, where the average run scoring factor wasn't 105. And the NL parks which Beckett pitched in were different than the NL parks that Sabathia pitched in. So when it comes down to it, park effect is so complicated that it's wise to just ignore their effects on competitions like the Cy Young Award.

Besides, Rays fans would have more a case for Delmon Young being robbed of a Rookie of the Year Award. Fenway Park's effect on right-handed batting average was almost as great as its effect on run scoring. And unlike the Cy Young Award where both competitors played in rather similar parks, Delmon Young's Tropicana Field actually was one of the least friendly parks for right-handed batting average. It was also a below neutral park for run scoring.

More than ERA, park factors affect the total runs that teams score. This is because ERA is a ratio stat, and the total effects of it are divided by 9. The same can be said about other ratio stats like H/9, BB/9 and K/9. Raw totals like Runs Scored are more skewed by park effect. If the Red Sox played in a neutral run scoring park in 2007, their runs scored total would look more like 802. This means that their runs scored total might not have ranked among the top five in the American League.

Again, this is difficult to calculate because eight out of the top ten run scoring teams in the American League last year played in parks which were friendly to scoring runs. The only two teams in the top 10 who were hurt by their park factor were the Rays and the Blue Jays.

The effect of Fenway Park on offensive production is so complicated, that I've broken it up into multiple parts. This article is only the beginning. Later, I will look at how Fenway Park affects average, home run totals, and errors. Many of these answers are sure to surprise you.

I know some of this information can be complicated, and not all of you have the Bill James Handbook handy as you read this. So if you have any questions, or need anything clarified, feel free to ask.


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