Talking Sabermetrics: The American League is Coming to a Ballpark Near You

We know the Astros are moving to the American League. But if you are like me, you probably are not as familiar with the AL, because you have watched mostly NL games. We know that the Astros have discussed the differences between the AL and NL style of play as they interview manager candidates. I started with the initial idea to make some statistical comparisons between the leagues---but then I realized that the statistics provide more areas of exploration than I originally envisioned. So, I may need to address the sabermetric implications of moving to the AL in multiple articles.

This article will address the most basic difference between the two leagues. In terms of the game, itself, there is only one rule difference between the leagues: the Designate Hitter (DH). However, this one difference spawns a number of potential divergences between the leagues. Because a DH bats instead of a pitcher, we know and expect that the AL will have better offensive stats than the NL. But, how much of this difference (if any) is due to a talent advantage in the AL, in addition to the impact of the DH rule?

And, if you are wondering why I chose a photograph of Lance Berkman: I am honoring many fans' desire to see Berkman as the Astros' first AL DH next year. However, Berkman recently poured cold water on the speculation, saying that it would be unlikely to happen. So, right now, it may be a fairly remote hope...but hold out hope if you like.

Studies of the NL vs. AL

A number of studies have attempted to quantify the difference in strength of talent between the AL and NL. Generally, the studies have concluded that the American League enjoys a significant talent advantage over the NL. However, most of these studies are based on earlier time periods and may not reflect a narrowing of the gap between the two leagues in the last few years, which I will address shortly. Analyzing talent differences requires creative methodologies, since the players play the bulk of their games in their own league, and the statistical differences can reflect offsetting strengths of pitching against hitting in the leagues.

Two 2008 Hardball Times articles by Derek Carty examined the statistical impact of switching leagues upon both pitchers and hitters. This article examined the effect upon AL pitchers who switch to the NL. This study supports the notion that AL hitting and (potentially) defense is better than the NL, benefitting pitchers who switch to the NL. The pitchers sustained significantly stronger K rates and lower RA and ERA by switching to the NL. The article on hitters who switch leagues suggests that overall pitching/defense in the NL is weaker than the AL. Hitters who moved from the AL to the NL generally enjoyed improvements in walk rates, singles, doubles, HR rate, and contact rate. The hitters had worse stolen base rates and triples after the move.

A 2007 Hardball Times article by John Walsh examined the difference in pitching/defense between the leagues by examining the OPS differentials for hitters who played in both leagues. The article concluded that hitters posted substantially higher OPS performances in the NL at a statistically significant level. Approximately 62% of the hitters performed better when they played in the NL, with 38% showing a better OPS when they were in the AL. The study provides support for the argument that the AL has better pitching talent.

Mitchell Lichtman (aka "mgl'), who would later co-authored "The Book," wrote a three part series for the Hardball Times in 2006 regarding the player talent differences between the AL and NL. He used several different methodologies to separate league hitting and pitching performance levels, and the studies are impressive, although dated at this point. Part I provide a global view of the issue; Part II examines the impact of player switching leagues; and Part III looks at NL batter vs. AL pitchers and AL batters vs. NL pitchers. He concluded that the AL enjoyed a hitting advantage equivalent to a 55% winning percentage over the NL. He also suggests that the AL might enjoy a pitching advantage that raises the winning percentage by 1% - 2% more. However, at the time he wrote the article, he observed that recent shifts in pitching talent had shrunk the pitching differences between the two leagues closer to equality. It appears to me that pitching advantages between the leagues may be volatile, since free agent signings, trades, and call ups of elite pitching prospects can make rapid changes in the quality of a league's pitching.

Current Differences in Leagues

These studies seemingly have dire implications for the Astros' AL move, in terms of the liklihood that the Astros' hitters and pitchers will suffer worse performance in the new league. And that may turn out to be true. However, all of the studies are based on previous time periods, ranging from 5 to 10 years ago. There is some evidence, accompanied by a frequently stated view, that the talent differences between the AL and NL have narrowed in recent years. This year, the AL in scoring two-tenths of a run per game more than the NL (4.45 R/G vs. 4.25 R/G), and presumably a substantial part of that difference is due to the existence of the DH. In particular the differences in league offensive performance in 2012 is markedly smaller than prior years, as the offensive environment has declined throughout the majors. I show the average OPS for the leagues this year versus the average for the six previous years, 2006-2011.

OPS Difference

(AL, NL, AL Advantage)

2012 .720, .730, .10

2006-2011 .739, .754, .15

Another way to analyze the leagues is to identify OPS differential which is purely related to the DH impact. If the DH effect doesn't fully cover the leagues' OPS differential, then the remaining difference may be caused by a talent difference. Admittedly, this is a somewhat superficial approach, but maybe it will be another piece of information, even if its usefulness is limited. For this analysis, I replaced the NL pitchers' OPS with a DH OPS, and recalculated the NL average OPS. I assume that the DH OPS falls somewhere between the average AL DH OPS and the average DH OPS for NL teams in interleague play. I examine 2012 and 2011. (I also satisfied myself that using a multiple year recent period wouldn't change the results to any significant extent.)

As you can see from the table, replacing the NL pitchers' plate appearances with a DH generally eliminates the AL OPS advantage, with the exception of 2011 for the case using the lower DH OPS for the NL. For the 2011 calculation, replacing the NL pitcher OPS with the NL Interleague DH OPS still leaves the AL with a slight OPS advantage. Although the result is interesting, and supports the notion that the DH itself is the primary cause of the current differences between the leagues, it may not prove anything. A defect of this method is that it doesn't consider the quality of pitching in the two leagues. If the AL has higher quality pitching than the NL, this would suppress the AL OPS, and possibly conceal an advantage in AL offensive talent. And other factors, such as the ballparks in each league, haven't been taken into account.

A major concern for the Astros is that the team's current players will suffer a decrease in their performance when they move to the AL. However, maybe the comparison, above, will allay those concerns to some extent. The pitchers are likely to get a bump in their ERA, just like other AL pitchers, because they face a DH. But that may be manageable, so long as the league switching effect is limited to the DH. Another takeway from this analysis is that the Astros are well advised to acquire a DH with sufficient offensive ability to exceed the average AL DH. Over the six year period 2006 - 2011, the average AL DH OPS was .784, compared to the average OPS of .717 for the NL interleague DH. The latter OPS would be similar to using NL outfield/1b bench players for the DH. AL teams clearly use a higher quality of hitter in that role.