Where did the offense go?

I was looking at the Major League batting leaders yesterday and noticed that 20 of the top 25 hitters according to OPS (on base plus slugging) were National Leaguers. The unlikely American League quintet among the leaders included Milton Bradley, Carlos Quentin, Josh Hamilton, Kevin Youkilis and Jack Cust. The batting average list also had only 5 AL batters in the top 25. Among the 22 batters with 10 or more homers, there were just 4 American Leaguers.

These leader boards looked very unusual so I wanted to examine the stats more closely. Table 1 below illustrates that the junior circuit is trailing the senior circuit in runs scored per game (4.37 in the AL versus 4.56 in the NL) through May 23. The last time the non-DH league scored at a greater rate than the DH league was 1974. The AL also trails in batting average, OBP, slugging and home runs.

Table 1: Offense in 2008 - AL versus NL

	AL	NL
R/G	4.37	4.56
BA	.259	.261
OBP	.329	.334
SLG	.397	.410
HR/PA	0.86	0.96

Table 2 below shows that offense is way down in the AL in 2008 compared to April and May of recent years. In fact, it's down a half run per game from last year - 4.85 in 2007 to 4.37 in 2008. The last time run scoring was that scarce for a full season was 1992. While batting averaged has dropped somewhat, the biggest declines have been in the power stats. Home runs are down 20% this year compared to the last three years while slugging percentage is down by about 6%.

Table 2: American League Offense from 2005-2008

	2005	2006	2007	2008
R/G	4.67	5.02	4.85	4.37
BA	.263	.271	.264	.259
OBP	.328	.338	.335	.329
SLG	.413	.433	.418	.397
HR/PA	1.00	1.15	1.04	0.86

So what's going on with the AL offense this year? There are a few different theories. In a recent column in The Sporting News, David Pinto of Baseball Musings illustrated that the American League is significantly younger than the National League this year. The average age weighted by plate appearances is 29.5 in the AL and 28.8 in the National League. Furthermore, the highest concentration of plate appearances in the AL have gone to players aged 31-34, an age when players are typically on the decline. Conversely, the highest concentration in the NL has gone to players between ages 24-30, the prime years for hitters.

In a Baseball Prospectus article, William Burke and Joe Sheehan looked at batted ball data and discovered that fly ball rates are down in the AL - .215 between 2005-2007 and .203 this year. Home runs per fly ball are also down - .124 between 2005-2007 and .111 this year. This suggests to me that pitchers are keeping the ball down more in an attempt to induce more ground balls and that is keeping power at a minimum.

Burke and Sheehan also suggested that teams are choosing defense over offense in their personnel decisions more now than they they did in the height of the "steroid era". I'm not sure I completely agree with them but a look at team defense reveals that the median Defensive Efficiency Ratio (percentage of balls in play converted into outs) has increased from .692 in 2006-2007 to .704 this year. Part of that is likely due to the increase in ground balls but BP article points out more possible evidence of better defense:

BABIP hasn’t changed all that much, but fly-ball rates have. Because the batting average on fly balls in play is generally lower than the BA on groundballs in play, a constant BABIP and a lower fly ball rate implies that defense has gotten better.

There has also been some speculation that reduction in PED use might have something to do with the drop in offense. Deadened balls are also a possibility. However, it seems that both of those factors would cause uniform drops across the AL and NL but only the American League offense has decreased. Thus, some of the other theories seem more plausible especially Pinto's theory about older players in the AL. Other possible explanations could be an emphasis on pitching that induces ground balls, and better fielding.