The Anatomy of a Good ERA

It has been pretty well documented that ERA is largely out of the control of pitchers; in fact, it might be determined less by the pitcher and more by his surrounding environment. I can’t say that with 100% certainty, but I would not be at all surprised.

With that in mind, let’s take a look at the factors that contribute to a pitcher having a low ERA compared with those of a pitcher with an uncharacteristically high ERA. The Nationals’ de facto ace in 2013 has so far been Jordan Zimmermann, who currently sports a 1.64 ERA backed by a 2.92 FIP. Our old buddy, Edwin Jackson, who signed a lucrative 4-year, $52 million contract with the Cubs last offseason, has a 3.23 FIP and a 6.39 ERA. Both pitchers seem to be pitching fairly effectively – why is Zimmermann’s ERA so much lower than Jackson’s?

Balls in Play
Zimmermann’s BABIP is .188; Jackson’s is .353. That is the equivalent 17 fewer hits against Zimmermann on 100 balls in play.  Behold the following chart, which shows BABIP for each batted ball type for each pitcher as well as the Major League average:

Player	Ground Balls	Fly Balls	Line Drives
J Zimmermann	.108	.073	.737
E Jackson	.203	.125	.926
MLB Avg	.235	.108	.680

Zimmermann is lower than Jackson across the board and lower than average in all but line drives.  It’s not very surprising when you consider that the Cubs are not a great defensive team; the Nationals are much better defensively.

Take a look at the line drives.  Through 6 games, Zimmermann has given up 19 line drives, of which 12 were singles and 2 were doubles.  Jackson has given up 27 line drives, of which 7 were doubles, 1 was a triple, and 17 were singles.  That’s a slash line of .926/.926/1.259 – or an OPS+ of 161 – against Jackson on line drives.  For whatever reason, Jackson has been hit very hard on line drives while Zimmermann has not.

Perhaps Zimmermann, who is known for his command, is able to induce weaker contact than Jackson.  Jackson walks more batters and generally has worse command than Zimmermann, which could make him more prone to mistakes.

Location, Location, Location

Check out these Heat Maps, courtesy of FanGraphs:

Zimmermann Heat Map vs. Righties

Jackson Heat Map vs. Righties

I clumped all of the pitches together.  Still, you can see that Jackson’s most concentrated area is dead, red, down the middle of the plate.  Zimmermann’s pitches, meanwhile, are more spread out; his most concentrated areas are not as dark as Jackson’s.  And they aren’t as close to the middle of the plate.  Furthermore – and I won’t distract you with more pictures – Zimmermann’s fastball has more natural movement in on righties and has averaged a mile per hour faster.  Throwing harder with more movement in better spots makes it more challenging for a batter to square up on a pitch.

Sheer, Dumb Luck

Zimmermann has stranded 83.3 % of baserunners in 2013; Jackson has stranded 52.3 %.  It’s hard to explain this with certainty, aside from Zimmermann walking one-third as many batters as Jackson, so maybe Zimmermann has just been luckier than Jackson.  Maybe the crazy Chicago wind has hurt Jackson, causing what would normally be an easy out to fall in for a hit (and inflate his BABIP at home to .470).

I think it’s pretty clear that Zimmermann is a better pitcher than Jackson, which plays a significant role in why he has a much lower ERA.  But even the best pitchers couldn’t maintain a lower ERA with no defenders.  And sometimes, the ball just does crazy things.