Other than solo home runs, a run scoring is almost always a team event that several players contribute to. Multiple hitters, multiple pitchers, and multiple fielders can be directly involved with whether or not a runner makes it around the bases. It's a reason to mistrust runs and RBIs for hitters and also a reason to mistrust ERA for pitchers.
Pitching is only a fraction of what prevents runs from scoring, with opposing batters, defenders, umpires, the ballpark, and some random luck being obvious contributors. We can assume that over enough time, the umpires, opposing batters, and luck will somewhat even out. But imagine pitching your entire career in Petco Park versus The Ballpark in Arlington. Imagine pitching your entire career with your outfield being manned by three Raul Ibanez clones versus three Mike Trout clones. You could see that having a pretty huge effect on a pitcher's ERA.
To get a better idea, I'm going to compare ERA to FIP and see what kind of correlation that has with defensive ratings. FIP -- calculated only from walks, strikeouts, and home runs -- is intended to be completely independent of defense. Because all balls in play are treated the same, it's hard to believe that that gives you a full grasp of a pitcher's abilities. It is intuitive that good pitchers would be better at inducing weak contact than bad pitchers. Here are some career batting average on balls in play against a few pitchers:
Roger Clemens: .284
Greg Maddux: .281
Randy Johnson, The Big Unit: .291
Roy Halladay: .292
Andy Pettitte: .308
CC Sabathia: .292
And a few more:
Darrell Rasner: .298
Scott Proctor: .277
Shawn Chacon: .275
Chad Gaudin: .308
Sergio Mitre: .308
Chad Qualls: .301
A cherry-picked list, but it wasn't hard to find examples. There might be a few Matt Cains that can live on the edge of the bell curve, but with very few exceptions, if you pitch long enough about 29% of the balls hit to the defense are going to be a hit. The successful pitchers are successful because they get strikeouts, don't issue walks, and don't give up home runs.
If we accept that FIP is a more direct measurement of the pitching contribution while ERA is a more direct measurement of the overall quality of the run prevention (pitching and defense), the two can be compared to demonstrate the defensive contribution. If a team's ERA is routinely higher than their FIP, you would conclude that the defense has likely been below average.
To measure team defense, I will average Ultimate Zone Rating and Defensive Runs Saved. So if you have a favorite between the two, it was included. If your favorite is neither, it's worth pointing out that team defense is a much more reliable measurement than individual defense*.
*If there's a ball hit to right center, is it Nick Swisher's ball or Curtis Granderson's? Did Granderson make a great play to range that far or did he just call off Swisher on a ball with a lot of hang-time? Do we have enough fly balls in that area to have a good idea of what's going on?
If we aren't trying to measure Swisher or Granderson individually, but the team as a whole, we don't care who catches the ball so long as it's caught. As we have a whole lot of data and a whole lot less gray area to to try to parse, things tend to be a lot more cut and dry.
The idea is that teams that have scored well by UZR and DRS would help their pitchers to a lower ERA. If this is true, we'd like to know the extent and who has been doing a good job of it. Below is a ten year aggregate (2003-2012) of defensive rating and ERA-FIP.
We can conclude that teams that rate well defensively are much more likely to match or beat a FIP projection. Conversely, teams that rate poorly defensively are likely adding several tenths of a point to their team ERA. You will have noticed that the Yankees have posted the worst defensive ratings in baseball, with much of that negative value accumulated in the earlier years that I looked at. Data that is more Yankee-centric:
This data suggests that in the past ten years, Yankees pitchers have had as much as 0.42 added to an expected ERA and as much as 0.28 taken off. That is a swing of 0.70 runs on factors outside the direct control of the pitcher. Imagine changing the team ERA from 4.00 to 4.70. That's changing from being the 2012 Rays to the 2012 Astros.
In the past few years, the Yankees have dramatically improved defensively. This is very likely to be another byproduct of a renewed focus on youth and a less aggressive approach in chasing veteran sluggers. Or it could just be Brett Gardner and the fact that the new slugging first baseman is a much better defender than the old slugging first baseman.