Modern baseball statistics (Part one: Park Factor)

Spring Training starts February 21st so the start of the MLB baseball season isn't that far away. I thought I'd take the time before the start of the regular season to explain, with a series of articles, some of the more modern statistics that announcers are using more frequently in broadcasts. I will introduce formulas when they are simple and I'll try keep them as easy to understand as possible. So let's get started with the stat called Park Factor.

Park Factor or PF is a measure to determine whether a baseball park is considered neutral, "pitcher friendly," or "hitter friendly." Put simply, it is a ratio of runs scored at home versus runs scored on the road for all games. A value of 100 is considered neutral or average. Any number above 100 gives the hitters the advantage and any number below 100 gives the advantage to pitchers.

It is useful to be able to compare Park Factors because, unlike other major sports, in baseball all stadiums are unique. Here are some values for some stadiums you might be familiar with: AT&T Park San Francisco 76, T-Mobile Park Seattle 85, Dodger Stadium Los Angeles 97, Wrigley Field Chicago 105, Kingdome Seattle 105, Arlington Texas 114, Coors Field Denver 138.

Why should you care about Park Factor? Notice the difference between the old Kingdome and the current T-Mobile Park in Seattle? The Kingdome was enclosed and at a consistent air temperature throughout the season. It was also a bigger baseball park with higher walls. So why is the current stadium a pitcher's park? The answer is the humid cool marine air coming off the Puget Sound. It's the reason why you don't see as many home runs as you used to in the Kingdome.

Want further proof? Let's look at a player who played a majority of his games with three teams: the Los Angels Dodgers, Seattle Mariners and Texas Rangers. I'm excluding the single season he played with Boston because the sample is too small to make a meaningful comparison.

Ages Team Avg/OBP/Slg (full season) HR HR/Year

19-25 Dodgers .274/.332/.463 147 21

26-30 Mariners .266/.317/.442 103 20.6

32-39 Rangers .304/.357/.509 199 24.8

The player is Adrian Beltre. Notice the drop-off in production during what should have been his prime years? He played 424 games at Safeco field during his career which he hit .253/.307/.407 with 56 HRs. Compare that to his games in Arlington. In 597 games he hit .329/.383/.555 with 120 HRs. Had he stayed with the Mariners he likely isn't considered for the Hall of Fame.

Other former Mariners such as Alex Rodriquez, Raul Ibanez, & Ichiro all had drop-offs in their production during their primes playing at Safeco Field. A few years ago, the Mariners moved in the walls to try to make the park more neutral but so far it looks like it isn't having much of an effect. I think more needs to be done including doing something, potentially, with the baseballs used in Seattle. There is a precedent for this in Coors Field where they use a humidor on all baseballs in an attempt to lower the park factor there that is the highest in the league due to the light air at the high altitude they play at.

Okay now to the formula for those interested.

Abbreviation Explanation

homeRS Runs scored by Mariners in Seattle

homeRA Runs scored by visitors in Seattle

homeG Number of home games

roadRS Runs scored by Mariners away from Seattle

roadRA Runs scored by Mariners opponents away from Seattle

roadG Number of road games

Park Factor = 100 *[ ((homeRS + homeRA)/homeG) / ((roadRS + roadRA)/roadG))]

Credit to Wikipedia for the equation and Baseball-Reference.com for the statistics.