Is the National League vs. American League Pitching Gap Expanding?

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Is the National League vs. American League Pitching Gap Expanding?
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The "Year of the Pitcher" in 2010 turned out to be the start of the "Age of the Pitcher." Good pitching has become easy to find in the last five seasons. And with the league's ERA starting the week at 3.80, it's been especially easy to find in 2014.

But if you want to find the really good stuff, here's a three-word hint: National League, yo.

Which, yeah, isn't entirely breaking news. The National League's pitching has been consistently better than the American League's pitching in the designated hitter era (since 1973), and the introduction of interleague play in 1997 has only closed the gap so much.

With an assist from FanGraphs, here's some perspective

AL Pitching vs. NL Pitching: 1901-2013
Span Average AL ERA Average NL ERA Difference
1901-1972 3.69 3.58 0.11
1973-1996 4.09 3.74 0.35
1997-2013 4.45 4.23 0.22

FanGraphs

This is as it should be. While AL pitchers spend most of their time facing AL lineups, NL pitchers spend most of their time facing NL lineups. That means regular matchups against pitchers, which is a lot of automatic outs.

Even still, things have been different so far in 2014. The gap between AL pitching and NL pitching is not only still there, but also a lot wider than usual.

From FanGraphs and Baseball-Reference.com come more numbers!

AL Pitching vs. NL Pitching: 2014
League IP K/9 BB/9 Opp. OPS ERA
AL 4977.1 7.71 3.33 .719 4.04
NL 5099.2 7.98 2.98 .698 3.58

FanGraphs/Baseball-Reference.com

Note: These and all numbers to come are current through Sunday, May 11.

It's the gap in ERA that stands out. That's a gap of 0.46 points.

Which is less than ordinary in the interleague era. Before this year, there had been only five seasons out of 17 in which the ERA gap between the two leagues was at least 0.30 points, and only once did it climb over 0.40 points (0.42 points in 1998).

It's not just ERA that says something funny is going on either. There's a gap between the two leagues' SIERAs—short for "Skill-Interactive ERA," which is a fancy-pants metric that strips luck out of the equation and estimates what an ERA should be based on things pitchers can control—of 0.28 points.

Since FanGraphs began tracking SIERA in 2002, that's the largest gap there's been between the two leagues. Behold an indication that, yeah, there really is something going on.

Now, the first thing that popped into my head was to check whether this is due to NL clubs beating up on one another. Surely they're not beating up on American League clubs in interleague play, right?

Actually, they are. 

Courtesy of MLB Advanced Media via MLB.com.

Per Baseball-Reference.com, the National League entered the week with a 3.76 ERA in interleague play. That's the best it's ever had in interleague play, but it's also not a figure out of the blue.

It's a continuation of what happened in 2013, as the NL dipped its interleague ERA to an all-time low of 3.96. That came with an all-time best .709 opponents' OPS, which has only risen to .725 this year.

This isn't happening at a time when American League clubs are falling behind National League clubs in terms of offensive production. By OPS, nine of the top 10 (via FanGraphs) offensive teams in the majors since the start of 2013 are American League clubs. And with interleague play now an everyday thing, it's not like NL clubs have been able to hide from these offenses.

This suggests that NL pitchers have gotten better at getting, you know, actual hitters out. 

And they have.

So much so, in fact, that they're now on par with AL pitchers. Courtesy of Baseball-Reference.com, here's a look at how the two leagues have fared against non-pitchers in the interleague era:

Data courtesy of Baseball-Reference.com.

Things were pretty much even in the beginning. Then there was a long stretch when AL pitchers held hitters to a lower OPS than NL pitchers. But in the last two years, things have become even again.

Point being: The results look legit. The overall suggestion is that National League pitchers have indeed gotten better.

And it's not just the results that say as much.

Courtesy of MLB Advanced Media via MLB.com.

One thing you might have noticed way back when is that NL pitchers are striking more batters out than AL pitchers in 2014. That's what you'd expect given the huge gap between the two leagues' swinging-strike rates this year: 9.6 to 8.9 in favor of the NL, according to FanGraphs.

The NL having the edge in either strikeout rate or swinging-strike rate isn't unusual, truth be told. But what is unusual this year is that NL pitchers have more of something that would create such advantages: velocity.

Baseball Info Solutions has been tracking velocity data since 2002. Since then, 2009 stands out as a key year, as it was the first season the league's average velocity climbed over 91 miles per hour (see FanGraphs). In the years since, it's stayed over 91 miles per hour.

That's typically been thanks to American League pitchers. But not this year:

AL Fastball Velo vs. NL Fastball Velo: 2009-2014
Year AL FBv NL FBv
2009 91.4 90.9
2010 91.5 91.0
2011 91.8 91.2
2012 91.9 91.3
2013 91.8 91.6
2014 91.4 91.6

FanGraphs

In 2013, the fastball velocity gap between the two leagues shrunk to just 0.2 miles per hour. And so far in 2014, it's National Leaguers who have been bringing the big heat for a change.

And how. Among the top 25 hardest-throwing starters in MLB, 20 are National Leaguers. Yes, 80 percent of the list. Holy four-letter word, Batman.

So that strikeout gap? Yeah, that adds up.

Another thing you might have noticed above is that NL pitchers have also been walking fewer batters than their AL counterparts. That, too, is no mirage.

Part of it can be traced to how NL pitchers have kept their first-pitch strike percentage over 60 in each of the last three seasons, whereas AL pitchers have yet to push their first-pitch strike percentage over 60 in the 2002-2014 window.

Beyond that, we can narrow our focus to the Age of the Pitcher and look at a stat called "Zone%" to see that NL pitchers have increased an edge in pitches in the strike zone that was established last year:

AL Zone% vs. NL Zone%: 2010-2014
Year AL Zone% NL Zone%
2010 46.8 46.3
2011 45.6 45.0
2012 45.3 44.5
2013 44.7 44.8
2014 46.4 47.3

FanGraphs

We just examined a pretty good explanation for why NL pitchers are so much harder to hit. Here's us looking at a pretty good explanation why they're also better at limiting walks.

In itself, this is a solid recipe for success. When you strike guys out, you're giving neither your defense nor the baseball gods a chance to beat you. When you're not walking guys, you're not beating yourself.

Of course, no pitcher can keep the ball out of play all the time. Guys are going to hit the ball between the lines, at which point a pitcher's fate is going to be completely out of his hands.

The best a pitcher can hope for is that batted balls are on the ground as often as possible, as ground balls:

  1. Often result in outs.
  2. Rarely go for extra-base hits.
  3. Get a lot of double plays.

Yeah, you can probably guess where this is going.

Batted-ball data on FanGraphs also goes back to 2002. Here's a look at the trajectory of the two leagues' ground-ball rates (GB%) since then:

Data courtesy of MLB Advanced Media via MLB.com.

While the AL's ground-ball rate has remained relatively steady since 2009, the NL's ground-ball rate has been trending upwards. And so far this year, the NL is working on its highest recorded ground-ball rate.

One thing we know about ground balls is that they're a lot easier to get when pitchers are throwing pitches other than four-seam fastballs. Harry Pavlidis found in a 2011 piece for The Hardball Times that four-seamers have easily the lowest ground-ball likelihood out of all pitches.

Lo and behold, the raw PITCHf/x data from FanGraphs suggests that NL pitchers have consistently thrown fewer four-seamers than AL pitchers since 2009:

AL FA% vs. NL FA%: 2009-2014
Year AL FA% NL FA%
2009 54.0 52.3
2010 40.4 38.4
2011 39.4 35.6
2012 35.7 33.1
2013 35.5 35.2
2014 35.4 34.4

FanGraphs

I'll stop short of calling these figures gospel due to how PITCHf/x has been known to confuse different types of fastballs. But if the general point that National Leaguers aren't throwing as many four-seamers is true, then we have a solid explanation for the league's upwards-moving ground-ball rate.

Which, in total, leaves us with a nice list of explanations for why NL pitchers have gotten to be so much more effective than AL pitchers. We started off looking at an ERA gap big enough to possibly be a fluke, but the assorted improvements of National League pitchers in recent years suggest otherwise.

Be warned that the situation isn't guaranteed to last forever. National League pitchers will always have a natural advantage as long as the designated hitter remains only in the American League, but those days could soon come to an end. Until then, there's nothing stopping AL pitchers from copycatting what's worked for NL pitchers, and they still have plenty of season left this year to do so.

But for now, anyway, the gap in pitching quality between the two leagues looks like the real deal.

 

If you want to talk baseball, hit me up on Twitter.

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