Quick: How long is a football field? OK, that’s easy – it’s 100 yards. So what are the dimensions of an NBA court? A bit harder, it’s 94 by 50 feet. One more: How big is a baseball field?

That one’s not so easy. Outside of all base paths being a uniform 90 feet, there’s an amazing amount of variation in the physical dimensions of the 30 major league ballparks, from the distance between home plate and the fences to the amount of foul territory. That variation has one huge consequence for analysts: If you really want to understand an individual player’s performance, you must account for where he plays.

The tool used to make those adjustments is called ‘park factor.’ We know just by watching that it seems like more runs are scored at the Colorado Rockies’ home park, Coors Field, than at the average park. But how many more? And in what ways is the park different?

The way to unlock this information is to compare what a team and its opponents do offensively in the team’s home park, especially in terms of scoring runs, and compare it to what that team and its opponents do on the road. Once you examine every combination of teams, you can do the math comparing different parks; you’ll find that Yankee Stadium, for example, slightly inhibits scoring compared to the average park.

It’s important to understand that park factors are completely independent of the teams involved. Having a great offense or a dominant pitching staff doesn’t change the park factor, because those performances are being compared across parks, not across teams.

By taking one more step, you can calculate what we call team park factors. The calculation for the Yankees would look something like this: 81x Yankee Stadium factor, plus 12x Fenway factor, plus 12x Camden Yards factor, etc. After factoring in the park effects for each of the games the Yankees played last year, then dividing by 162, you would come up with their team park factor. You can use that factor to adjust a player’s season up or down to a theoretical average park.

Todd Helton, to take one prominent example, has hit .327 AVG/.456 OBP/.615 SLG this season, giving him the second-best On-Base Plus Slugging (OPS) in baseball, behind Barry Bonds. But Helton plays in the poster child for park effects, Coors Field.

The Rockies’ team park factor of 1140 this year (where 1000 represents the average park) is by far the highest in the game and means that Helton’s stats have been inflated by 14%. Once you do that math, as well as adjust for the league (that’s a separate topic), you find that Helton’s “true” line for the season, without the influence of Coors, is .299/.428/.560.

This isn’t to denigrate Helton’s considerable offensive talent; it’s just a way to make sure our picture of a player isn’t skewed by his environment.

Park factors operate on the macro level, but they’re built on the micro level. Let’s break down some numbers from Coors Field so far in 2004:

Numbers greater than 100 mean that a park favors an event as compared to the average, while a number below 100 means a park puts a damper on certain events. These are calculated by looking at the performance of the Rockies and their opponents at Coors, and comparing them to the performance of the Rockies and their opponents away from Denver. You compare the two sets of numbers, and express it as a ratio, with 100 as the midpoint where the numbers are the same both at home and away.

With the park factors we looked at earlier, we know that Coors Field is a hitter’s haven. But this breakdown lets us figure out exactly how it changes the game. There are 19% more hits there than in other stadiums. We think of home runs being a big facet to the scoring explosion at Coors, but take a look at the numbers for doubles and triples: a 25% increase in doubles, and a massive 99% jump in triples.

Why so many doubles and triples? It’s easy to see if you take a look at the layout of the park. To try and keep home runs within reason in Denver’s mile high air, the fences are set miles away from the plate. The fence is 347 feet away down the left-field line and 390 feet away in left-center, before bulging out to 415 feet in straightaway center field. In right-center, it’s 375 feet, and 350 feet down the line in right.

Those distances – bigger than any other park in the game, especially down the lines – create a gigantic outfield with more ground to cover and more places for balls to get past outfielders. Voila! More doubles and triples.

On the other end of the charts is the new home of the San Diego Padres, Petco Park:

Petco has a bigger negative effect than Coors Field has a positive one. The wall in right-center is 411 feet away from home plate, which has caused dozens of left-handed sluggers to toss away their bats in frustration as they watch a ball which would be out in any other park die on the warning track.

And if you’re a righty, the news isn’t much better: left-center is 402 feet away. The heavy marine air around the park also plays a key role in restricting homers, especially compared to the thin mile-high air at Coors. So while San Diego’s Jake Peavy sports a glittering 2.14 ERA at this point in the season, once you factor in his environment, that becomes a 3.15.

Journalists are taught to answer six questions: Who? What? Where? When? Why? and How? In the world of baseball statistics, who and what are easy questions to answer, but if you forget to account for the where, you’ll never know the true value of a player.

This article was provided by Baseball Prospectus. The Sun will run exclusive content from Baseball Prospectus throughout the 2004 season. For more state-of-the-art baseball content, visit www.baseballprospectus.com.