A Random Walk in San Juan

Ballparks affect ballgames. Sometimes the effect is obvious, such as that of Coors Field; other times it is subtle, such as most other parks. But it’s real, and it impacts the game we follow so obsessively. To illustrate the point, let’s numerically wander around a few of them.

The numbers I am going to use are simple park factors; a ratio of runs scored by both teams (home and visiting) in each ballpark, compared to games the home team played on the road. For example, we’ll take all the runs scored (by both teams) at Miller Park in Milwaukee, and compare them to all the runs scored (by both teams) in all the other parks the Brewers played in. And we’ll do that for all ballparks this year and last year. What the heck, we’ll do the same thing for home runs, too.

This is a crude way of measuring ballpark impacts. In fact, ballpark factors are an entire field of study in and of themselves, but I’ll leave the more detailed analyses to my more qualified compatriots.


One of those compatriots, Joe Dimino, took a close look at Citizen’s Bank Park earlier this year. He found that Citizen’s has increased scoring somewhat, but increased home runs a lot. Here’s a quick look at some updated numbers (through July 22nd). I’ll also include the 2003 park factors for the Vet, for comparison’s sake.

                      2003    2004    Diff
Run Park Factor       0.90    1.06    0.16
HR Park Factor        0.88    1.26    0.38

The trend that Joe noted before has continued, albeit at a more muted rate. You might want to keep these differences in mind when comparing this year’s Phillies’ team to last year’s.

In general, fans tend to think that a big increase in home runs means a big increase in runs scored. That’s not really true at Citizen’s, and it is definitely not true at a ballpark that made its debut last year.


In its inaugural year, the Great American Ballpark quickly gained a reputation as a home run park, although teams in general didn’t see their overall scoring increase. In other words, the increased home runs were offset by a lower number of runs from all other hits. Let’s see how things have gone in 2004:

                      2003    2004    Diff
Run Park Factor       0.99    0.85   -0.14
HR Park Factor        1.22    0.98   -0.24

This year, the story is somewhat different. Although there is still a difference between home runs and other types of runs, the home run rate is now about even with all other ballparks. And the overall scoring rate is one of the lowest in the majors. There’s a cautionary tale here: a ballpark is a fickle mistress and less predictable than we’d like. I’ve got some more examples below. But first, the other new ballpark in town.

San Diego

The Padres opened PETCO Park earlier this year, as reported by our man on the scene. The Padres were already used to a low scoring ballpark, but things have gotten ridiculous this year.

                      2003    2004    Diff
Run Park Factor       0.82    0.75   -0.08
HR Park Factor        0.81    0.68   -0.13

When it comes to scoring a run, or hitting a home run, PETCO has been the most difficult ballpark in the majors this year. Its location on the ocean may have something to do with this, though I’m no meteorologist. But those fine pitching stats posted by the likes of Jake Peavy and David Wells should be interpreted with a grain of salt.

Still, the impacts of these new ballparks pale in comparison to the impact made by a change in a park in the Midwest, where all they did was move the fences out a bit.

Kansas City

                      2003    2004    Diff
Run Park Factor       1.28    0.86   -0.42
HR Park Factor        1.09    0.69   -0.40

There have been two huge changes in ballpark factors this year, and Kansas City’s is number two. During the offseason, the Royals moved the fences out ten feet (after having moved them in in 1995) and the difference has been huge. Prior to 1995, Kauffman Stadium was about an average run-scoring ballpark. When they moved the fences in, it became the biggest run-scoring park in the AL. Now it’s Shea and Chavez Ravine rolled into one. Why the difference this year? Who knows?

Baseball Coding with Rust – Intro
Getting into the nitty gritty of baseball action via a new(er) programming language.

Want to know where the number one change in ballpark impact has occurred?


Ever hear of the book A Random Walk Down Wall Street? In it, Burton Malkiel elaborated on the theory that you cannot tell the future direction of a stock, or the stock market, by its past performance. The past is meaningless, the theory goes. The future direction of the market is random, relative to its past.

Sometimes ballparks feel this way, too. Most years, the impact of a ballpark is predictable. There are other years, however, when the impact of a ballpark feels like a random walk. Case in point: wherever it is the Expos call home:

                      2003    2004    Diff
Run Park Factor       1.38    0.88   -0.50
HR Park Factor        1.54    0.84   -0.70

The Expos have a fantastically bad offense this year, and some of it appears to be due to home cooking. While the opposition bats have been somewhat subdued in Expo home games, the Expo bats have been downright comatose, yielding an extreme difference in ballpark impact this year compared to last year.

But what is an Expo home game, anyway? Sure, most teams’ home parks are in the town they are named for, but not Montreal’s. They have two “home parks:” Olympic Stadium in Montreal and Hiram Bithorn Stadium in San Juan.

Remember the spectacular high-scoring games at Hiram Bithorn Stadium last year? Well things have changed this year. Let’s compare the stats from games at Bithorn against those at Olympic, by totalling the runs scored by both teams in each game:

                      Games   Runs  Runs/G    HR   HR/G
Hiram Bithorn Stadium    21    134     6.4    24   1.14
Olympic Stadium          24    203     8.5    60   2.50

Actually, it is the Expos who suffer the most in San Juan. They have averaged only two runs a game in Bithorn, and they’ve managed to hit only four home runs in 21 games. The Expos’ opponents have scored about an average number of runs at Bithorn. You can ascribe all the causal elements you want to this phenomenon. In fact, you could easily turn this into another scathing indictment of MLB’s stance toward the Expos. But I’ll take the random walk.


Here is one more example of the inconstancy of ballparks. Remember when Camden Yards first opened and everyone thought it was going to be a “hitter’s paradise?” Well that myth has already been discredited many times, but maybe they just had the year wrong.

                      2003    2004    Diff
Run Park Factor       0.90    1.12    0.23
HR Park Factor        1.04    0.95   -0.09

This year, Camden Yards has been a hitter’s haven, if not exactly a paradise. In particular, opponents’ bats have lit up Camden this year, or maybe it’s just been the Orioles’ pitchers dimming down.

You know, I really do believe in the impact of ballparks. It’s just that they are inconsistent and hard to predict, even over an entire season. But just to reassure you, I’ll present one of the “sure things” among ballpark impacts, right here in my hometown.


Who wants to call a ballpark U.S. Cellular? No one I know. Most people in Chicago call it “the Cell,” but I prefer to call it “the Joan,” after Joan Cusack, U.S. Cellular spokeswoman and Chicago-area resident. Not my idea, by the way, but a good one.

So, what about Joan, you ask? Well, the Joan has been the easiest place to hit a home run this year. In fact, the Joan has been a home run park for quite a while. Here are the Joan’s home run park factors for the last four years:

2001:  1.26
2002:  1.20
2003:  1.33
2004:  1.30

It appears that the Joan is more steadfast than fickle. Home runs have been flying out of that park for quite a while. In fact, the easiest place to hit a home run this year has been Chicago. Here are the top five home run parks for 2004:

CHW: 1.30
CHC: 1.29
TOR: 1.28
PHI: 1.26
COL: 1.25

By the way, last year’s home run park factor at Wrigley, where wind direction can have a significant impact on batted balls, was .994. Oops. There I go again.

References & Resources
Thanks again to the providers of the data, Baseball Info Solutions. The U.S. Cellular home run park factor was recently well-covered by the Chicago Tribune. And you can find park factors throughout the year at ESPN’s site.

After I published this article, ballpark expert Kevin Johnson sent the following citation regarding offseason changes at Bithorn Stadium, which I should have included (if only I had known!):

Hiram Bithorn Stadium has a new look and Bobby Cox approves. Last year’s hard artificial turf has been replaced by Field Turf, which better duplicates real grass, and the outfield fences have been moved back 10 feet to Montreal’s Olympic Stadium dimensions. The result has been a drastic reduction in home runs.

“I think it was much easier to hit one out last year and the new carpet has made it slower,” Cox said. “I think it’s good. Last year it played a little small.”

The increase to 404 feet to center and 375 feet to the power alleys isn’t the primary reason balls are no longer flying out of the stadium. The trade winds are blowing in, knocking balls down and preventing the Expos from even reaching the warning track with most of their drives.

Source: The Macon Telegraph. Thanks, Kevin!

Dave Studeman was called a "national treasure" by Rob Neyer. Seriously. Follow his sporadic tweets @dastudes.

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