The Coors Field Effect & the Jason Jennings Trade

*Update at end of article

This article comes in response to some of the best discussion and analysis of a trade that I've been a part of lately, (read the discussions, here and here). I've decided to dive into some stats in hopes of projecting Jason Jennings' 2007 performance as well as find out how much Coors Field actually does affect a pitcher's ERA.

First of all let me say that despite popular belief, the infamous humidor was actually installed at Coors Field in 2002. This apparatus received a lot of press last year due to the Colorado Rockies pitchers' amazing performance at home. However, as I said, the humidor has been in place since 2002. To be honest, I think the Rockies simply finally had a pitching staff that suited their stadium, not the other way around. Now that we've got that out of the way, let's get into some of the stats.

I've gathered a somewhat small list of pitchers who have pitched in Colorado and looked at their ERA before, during, and after their time spent with the Rockies. While the list is far from complete, it's the best I can do while still holding down my day job. I'd love to do this stuff for a living but we're not there yet. Without further ado, the pitchers and their stats...

Joe Kennedy - in 2003 with Tampa Bay he posted a 6.13 ERA. In 2004 with the Rockies, that dropped to 3.66. And oh, by the way, his ERA was 0.14 BETTER in Colorado than away from it. In 2005 Kennedy's ERA ballooned up above 7 with the Rockies. He was then dealt to Oakland where his ERA dropped to 4.45 after the A's converted him to a reliever. His career ERA now sits at 4.79.

Pedro Astacio - In 1997 Astacio went from the Los Angles Dodgers to the Colorado Rockies. His ERA with the Dodgers was 4.10 compared to his 4.25 ERA with the Rockies. Over the next 4 years Astacio's ERA with the Rockies averaged about 5.50. He then dropped his ERA to 3.14 when he went to Houston, however that was only over the course of 4 starts. The next year with the Mets, Astacio's ERA rebounded to 4.79, much closer to his career average of 4.67.

Daryl Kile - Kile had the best year of his career in 1997 with the Houston Astros. It should be noted however, that for the three seasons before '97, Kile's ERA averaged 4.57 with the Astros. In 1998, Kile went to Colorado and proceeded to put up ERA's of 5.20 and 6.61. In 2000, Kile went to the St. Louis Cardinals and resurrected his career averaging an ERA of 3.57 for two and a half years, before his life and career were tragically cut short by heart problems. His career ERA was 4.12.

Jose Mesa - In 2005 Jose Mesa put up a 4.77 ERA with the Pittsburgh Pirates. Last year with the Colorado Rockies, Mesa finished the year with a 3.86 ERA.

Kevin Ritz - From 1990 to 1992, Kevin Ritz had an average ERA of 9.46 with the Detroit Tigers. Ritz then went to Colorado and over his first three years there averaged an ERA of 5.03.

Mike Hampton - In 1999, Mike Hampton had a career year for the Houston Astros. He compiled an amazing 22-4 record with a 2.90 ERA. The year after, Hampton had a 3.14 ERA with the New York Mets. Similarly to Kile, Hampton then put up two horrible years with the Colorado Rockies. Hamptons ERA's over 2001 and 2002 were 5.41 and 6.15 respectively. Hampton then escaped Colorado, moving to the Atlanta Braves putting up a much improved ERA of 3.84 and 4.28 over the next two years. It's undeniable evidence that Coors Field kills pitchers right? Hampton gave up career highs in home runs over those two years, so that has to be Coors right? Not so fast. Hampton also had career lows in strike outs per 9 innings as well as walks per 9 innings pitched.

AND, the clincher is that Hampton's ERA was actually 0.76 WORSE away from home in 2002 (his 6.15 ERA year). Even more astonishing is the fact that 12 of Hampton's 15 losses that year came away from home. By those numbers, Coors Field actually HELPED Hampton's ERA.

So, what have we learned? Well, Daryl Kile and Mike Hampton sure seemed to suffer from pitching in the thin air of Coors Field. However, as we saw with Hampton, you can't attribute their inflated ERA's merely to where they pitched. Mike Hampton would have had a horrible year in 2002 if he had pitched at sea level. To top it off, I have the stats to back that statement up! As we just saw, Hampton's ERA was three quarters of a run better at home than when pitching away from Coors Field. So, let's go back and take a look at the pitcher who inspired this whole article, Jason Jennings.

Jennings is coming off a career year in terms of ERA. His 3.78 ERA was nearly a run better than his career average. So, will leaving Coors behind for the spacious center field of Minute Maid Park help Jennings' stats? Before you answer take a look at Jennings' stats over the last three years. His ERA has averaged 4.71 but is nearly half a run (0.43) better away from Coors Field. But wait, his ERA was only 3.78 last year, if he drops half a run off that he'd have a very good 3.25 next year for the Astros. Well, if you only look at last year's stats, I've got some bad news for you. Jennings' ERA was actually nearly half a run (0.41) worse away from home.

So, where does all that leave us (besides probably half asleep)? We've learned that you can't simply subtract a run or more from a pitcher's ERA just because he pitches half his games at Coors Field. In fact, several pitchers, including Mike Hampton, performed BETTER at Coors field than away from it. Last year, the Houston Astros' newest pitcher, Jason Jennings, fell into that category. So, using last years numbers, his ERA for Houston should be around 4.00. If you look at Jennings' stats for the previous three years, his ERA will be about half a run better away from Coors but that is only good for an ERA of about 4.25. So, looking at those numbers I'll let you decide whether you'd want Jennings as your team's #2 pitcher.

Update: I just found Kile's split stats for his two seasons with the Rockies. His ERA was nearly 1.5 and 2.0 runs better AWAY from Coors than in it for his two season with the Rockies. It would certainly appear that Coors field had a negative impact on his ERA. However, Kile appears to be the only pitcher who's stats support the idea that Jennings will be a drastically better pitcher away from Colorado. I'm still digging into the stats but I'll post more when I have it.