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October 23, 2007

Prospectus Toolbox

A Tale of Two Ballparks

by Derek Jacques

Lots of people are excited to see the Colorado Rockies in the World Series, but statheads are probably watching this series with a special glint of joy in their eyes. You see, for the past fifteen years, the Rockies have been the focus of one of the great inquiries in baseball-how do you win at altitude? Performance analysis can be defined as the study of baseball in context, and since 1993, the city of Denver has given the major leagues one of the most fascinating contexts in its history: a relentless high-run environment.

Sure, there have been extreme run-scoring environments throughout the game's history, but usually those have resulted from the vagaries of ballpark construction, especially parks built too big or too small. Often, the situation could be remedied (or at least adjusted) by moving the outfield fences in or back, or adding or removing rows of seats. In contrast, Denver's run-scoring environment is more a matter of atmosphere than of ballpark dimensions. Higher altitude means thinner air, and thinner air means less friction to slow down thrown or batted balls, or to make curveballs curve. The conventional wisdom was that you could move the fences around as much as you like, but absent a pressurized dome, you weren't going to have a normal game of baseball in Colorado.

So the questions that follow from that environment, with a ballpark effect this extreme, have been how to take advantage of it, or how not be prejudiced on account of it. In his tenure as Colorado GM, Dan O'Dowd has famously tried a number of different tactics-obtaining contact hitters, or changeup pitchers, or big whiffers with power-often with little to no positive results. Until this year, baseball in Colorado was a bit like the movie March of the Penguins-an often-heartbreaking story of a dedicated group struggling to survive in a hostile environment.

With the Rockies facing another team, the Boston Red Sox, whose home ballpark is considered a unique environment-and an environment that has from time to time been considered a possible hindrance toward building a successful ballclub-this seems like a good time to take a refresher on the basics of ballpark effects. I'm also throwing in an extra-long "Further Reading" section, so that you can review some of the things that we've written about baseball at altitude.

The concept behind park factors is pretty simple. By comparing the home and visiting team's runs scored or allowed in and away from a given ballpark, you can calculate how much the ballpark increases or suppresses run scoring, above the league average. While the concept and the inputs are pretty simple, the actual calculation is rather challenging, and made more so by unbalanced schedules and interleague play. Park factors are habitually arranged on a 100 scale, where numbers above 100 indicate a percentage of scoring above average, and numbers below 100 indicate a score-suppressing effect, relative to the average. You'll also notice that batter's park factors (BPF) and pitcher's park factors (PPF) are calculated separately, and sometimes diverge by a few points.

Let's look at the ballpark factors of the two World Series sites over the years:


Colorado
Year    BPF     PPF
2006    108.3   108.3
2005    111     111
2004    120     117
2003    112     111
2002    121     119
2001    122     119
2000    131     128
1999    129     126
1998    119     120
1997    123     123
1996    129     129
1995    128     128
1994    116     118
1993    120     122

Boston
Year    BPF     PPF     Year    BPF     PPF
2006    101.7   101.7   1991    105     105
2005    103.2   103.2   1990    105     104
2004    106     105     1989    106     106
2003    105     104     1988    104     104
2002     98      99     1987    103     102
2001    101     101     1986    100     100
2000    101     101     1985    104     103
1999    105     104     1984    105     104
1998    100      99     1983    107     107
1997    102     102     1982    106     106
1996    103     103     1981    106     106
1995    102     102     1980    105     105
1994    105     105     1979    106     105
1993    108     108     1978    111     109
1992    107     107     1977    112     111

As you can see, the numbers fluctuate from year to year, and the range of fluctuation is substantial, more than ten points over the time that MLB has been in Colorado, and twelve or so points over a thirty-year period in Fenway's history.

Why do these fluctuations happen? It helps to keep in mind that since you're measuring against the league average, the target is often moving-other teams change ballparks, plus there's expansion and realignment-but a certain amount of randomness also factors in. To reduce the effect of these fluctuations, park factors are usually measured in multi-year averages, where possible.

That said, 2005-2006 seems to represent a serious change in the trend for Coors Field, a reduction from "an unbelievably good place to score runs, like hitting on the moon" to merely "a very, very good run-scoring environment." In this regard, the numbers don't tell us why the change has happened-park factors are results-oriented. The accepted wisdom is that there's a "humidor effect" from the temperature- and humidity-controlled conditions in which baseballs are currently stored in Colorado, with the assumption that's what is suppressing the park's run-boosting abilities, but the park factor can only tell you that run-scoring in Denver is down, not why.

Increased ball velocity and trajectory data may some day give us some of those answers. Dan Fox, for example, has done some very promising work with pitch F/X data and weather conditions. Nonetheless, because of sample size issues there is no defined park effect for "when the wind is blowing out at Wrigley" or how very late October weather will affect how Fenway, or particularly Coors, will play.

Finally, as some of you might remember from the interminable Non-Contact series, we don't need to restrict park factors to just runs scored and allowed. The same methodology can be used to determine if a ballpark suppresses or increases strikeouts, walks, home runs-basically, any statistic-above or below the league average. Also, we can look at the different effects the ballpark has on left- or right-handed batters, which is a good idea when dealing with an asymmetrical ballpark like Fenway:


           HR      1B      2B      3B      BB        K
COL RHB   108.4   108.6   105.7   127.3   100.3    93.4
    LHB   105.3   107.5   104.1   113.7   103.7    94.1
BOS RHB    95.3    99.3   113.4    86.4    99.9   101.5
    LHB    90.3   100.8   112.8   107.0    99.2    98.6

It's mildly surprising that Coors Field seems to give right-handed hitters more of a boost over their lefty counterparts than Fenway does-the Green Monster's main benefit to right-handers is a significant boost in home runs.

William Burke and Jason Pare contributed research to this article.

Further Reading

Keith Woolner, "How Much Does Coors Field Really Matter?" in Baseball Between the Numbers (Basic Books, 2006): Everything you wanted to know about baseball at altitude but were afraid to ask.

Joe Sheehan, Daily Prospectus, "Park Effects": The basics of park effects.

Total Baseball: For those of you who want to do the math, the glossary of this site contains a formula for calculating park factors (as well as a number of other sabermetric treats).

James Click, Crooked Numbers, "More Time in the Park": James takes aim at the "moving target," the effect of new teams and ballparks on the league average, that I mentioned above.

Tom Tango, The Book Blog, "Run Impact in Parks": The comments to this post, particularly show some creative ways that component park factors can interact to create a better picture of the ballpark.

Dan Fox, Schrodinger's Bat, "Advancing in Context": A look at park factors and baserunning.

Clay Davenport, "Park Factor Review": This 2004 article from BP's park factor maven runs down every team from the majors to the South Atlantic League.

Rany Jazayerli, Doctoring the Numbers, "Triples, Colorado, and Cristian Guzman": This 2000 article discusses Colorado's triple-boosting effect.

Rany Jazayerli and Keith Woolner, Doctoring the Numbers, "Coors Field" and "Coors Field Revisited": In this pair of 2000 articles, Rany and Keith examine and debunk the so-called "hangover effect" of playing at high altitude.

Rany Jazayerli, Doctoring the Numbers, "Offense in Colorado" and "Defense in Colorado": Further explorations of park effects and their discontents at altitude.

Dan Fox, Schrodinger's Bat, "Swing and Miss" and "More Humidity": These 2006 articles take on the humidor effect, weather, and the length of the grass at Coors.

Joe Sheehan, Prospectus Today, "Wet Horsehide" and "Something's Rotten in Denůver": A different view on using the humidor.

Derek Jacques is an author of Baseball Prospectus. 
Click here to see Derek's other articles. You can contact Derek by clicking here

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