Putting Coors Field Generated Statistics Into Context

As the Diamondbacks travel to Colorado to play this weekend, we revisit park adjusted metrics and how to use them

The first place Arizona Diamondbacks (14-12) travel to Colorado to play the last place Rockies (8-18) at Coors Field this weekend for a three game series. The first game starts tonight at 5:40 P.M. MST.  The temperature is expected to be in the mid 50's at game time, dropping to low 50's or even into the high 40's by the end. Saturday and Sunday are expected to be warmer however. 

Coors Field is always a challenging ballpark to play in, no matter the current state of the Rockies. Offense is aways inflated in the thin air of 5000 feet elevation. This makes it difficult for visiting teams by putting strain on their pitching staffs both during and immediately after a trip to Colorado. This run environment also impacts the perception of the Rockies as a team. Their pitching statistics usually make their pitchers look worse than they really are and the hitters usually aren't as good as they appear on the surface. So how can we properly contextualize the statistics like ERA or OPS we usually rely on to evaluate a team?

Fortunately there are tools to do so. Baseball-Reference.com park factors  are the starting point. That number rates Coors Field at 115, which means that the park favors the hitters by roughly 15% compared to league average. 

How does this impact the basic numbers we might use to evaluate a team? The short answer is a lot. If one were to look simply at the Rockies team ERA of 5.09, which currently ranks 26th in MLB, they might conclude that the Rockies pitching staff is one of the worst in the majors. At the same time, the Diamondbacks team ERA is nearly a half run lower, at 4.62, and ranks 22nd.  One might also conclude that the D-backs pitching staff has been better than the Rockies. After all, half a run difference in ERA tells you all you need to know, right? Well..not really.  

Baseball Reference created a metric based off the above mentioned park factors called ERA+.  This metric measures ERA against the league average, with the basic formula of 100*(League Avg / ERA). It then adjusts for the pitcher's ballparks, or park factors.  100 = league average, the higher the number, the more it favors the hitter. The lower the number, the more it favors the pitcher.  

In this metric the Rockies team ERA+ is 98, ranking 18th in MLB. The Diamondbacks team ERA+ is 95 and ranks 22nd the same as their ERA rank.  At this point it's important to say that the park factors are not entirely precise. They give a reasonable approximation. The main takeaway here should be that despite the Rockies having an ERA nearly a half run higher than the D-backs, the two teams are actually in the same tier when it comes to run prevention, and actually have been for the better part of the last 10 years.

The flip side of this is that the Rockies offense is frequently overrated. That's simply because their hitting stats get inflated by Coors Field. So anyone just looking at batting average, or OPS without park adjustments might think their numbers are above average in most years. (Even without adjustments the Rockies offense is clearly among the worst in 2023 so far, averaging just 3.77 runs a game). For example since 2019 the Rockies team OPS is .737 and ranks 12th in MLB. The D-backs .713 OPS during that span ranks 23rd. But once you adjust for ballpark factors, the Rockies 87 OPS+ ranks 24th while the D-backs 92 OPS+ ranks 22nd.  Report link

It should be noted that Chase Field used to play as a good park for hitters in most seasons. But since the introduction of the humidor in 2018, and the artificial turf in 2019, have made the park very close to neutral, and perhaps even slightly favoring pitchers. The current multi year park factor for Chase Field is 99 vs. league average 100.

The other primary statistics website that many people like to use for the park and league adjusted stats is Fangraphs. Their metrics are slightly different. For ERA they use ERA-.  It's the same concept, but due to a different formula used, the lower the number the better it is.  They also use slightly different park factors than baseball reference. Currently the Rockies have a 105 ERA-, meaning they.  calculate the Rockies run prevention to be about 5% worse than average. The D-backs have a 107 ERA-, or 7% below average. 

For offense Fangraphs uses wRC+, or weighted runs created plus.  The Diamondbacks rank 19th with a 96 wRC+, while the Rockies'  72 wRC+ ranks 29th.  Despite these minor differences in method of calculation and ranking, one can glean the overall concept as being the same.  Don't be mislead by Rockies pitching and hitting numbers. Their pitchers are likely better than you think, and opposite is true for their hitters.  Most fans intuitively know this, but hopefully these tools shown today will help give a clearer understanding. 

Just like you probably don't know exactly how your computer or T.V. works, one doesn't need to know all of the math and formulas to utilize these tools.  OPS+ and ERA+ are easy to find on each team and player page at Baseball Reference, as are wRC+ and ERA-  at Fangraphs.  The basic idea of centering average around 100 and viewing it as a simple number above or below 100 to understand the team performance relative to league average is easy to grasp and easy to access.