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November 13, 2009

Prospectus Hit and Run

Digging the Long Ball

by Jay Jaffe

One of the six or seven factoids about the World Series which somehow escaped my notice when writing this site's epic preview was that the matchup between the Yankees and Phillies marked the first time since 1926 (Yankees vs. Cardinals) that the two teams who led their respective leagues in home runs faced off in the Fall Classic. The Phillies hit 224 homers and featured a quartet of hitters—Ryan Howard, Chase Utley, Jayson Werth, and Raul Ibañez—who each hit at least 30, the 12^th such combo in history. The Yankees swatted 244 homers and had seven players with at least 20 homers, the fourth team with such a widespread distribution of dingers.

Given those numbers, I had intended to set aside time to follow up my late-April look at home-run rates, particularly with Nu-Yankee Stadium's homer-promoting qualities the subject of so much discussion this year. Alas, it slipped through the cracks, but that only means we've got something else to warm our hands with as the Hot Stove season kicks off.

Back in April, amid so much breathless commentary from the mainstream media regarding rising homer rates, I noted that on a per-game basis, homers were up a whopping 20.8 percent relative to the previous year's comparable timeframe (the late start in the year made an April-to-April comparison less valuable), and 7.7 percent relative to overall 2008 rates. That trend leveled off, though as in years past, April's data foretold the overall direction of change. Looking at overall rates in the context of the long ball-happy Wild Card Era:

Year    HR/G  Change
1995   1.012   -2.1%
1996   1.094    8.2%
1997   1.024   -6.4%
1998   1.041    1.7%
1999   1.138    9.3%
2000   1.172    2.9%
2001   1.124   -4.1%
2002   1.043   -7.2%
2003   1.071    2.8%
2004   1.123    4.8%
2005   1.032   -8.1%
2006   1.109    7.4%
2007   1.020   -8.0%
2008   1.005   -1.5%
2009   1.037    3.3%

I've expressed the per-game rate as per team per game, a preference that jibes with our tendency to talk of run-scoring environments in terms of a single team (as in normalizing statistics to a 4.5 RPG environment). Overall, home runs increased by 3.3 percent over 2008, a rather ho-hum change that's notable primarily because 2008 marked the lowest rate of the era, with 2007 the being second-lowest. Excluding the shortened 1995 season, this year's rate ranks 10^th in the current era.

As it turns out, that modest increase was actually the result of sharply divergent trends in the two leagues:

Year     NL    change    AL    Change    Gap
1995   0.952   -0.2%   1.071   -3.7%    0.119
1996   0.979    2.8%   1.210   13.0%    0.231
1997   0.954   -2.6%   1.094   -9.6%    0.140
1998   0.988    3.6%   1.102    0.7%    0.114
1999   1.117   13.0%   1.163    5.6%    0.046
2000   1.159    3.8%   1.187    2.0%    0.028
2001   1.139   -1.7%   1.106   -6.8%   -0.033
2002   1.003  -12.0%   1.088   -1.6%    0.085
2003   1.046    4.3%   1.101    1.2%    0.055
2004   1.099    5.1%   1.150    4.4%    0.051
2005   0.995   -9.5%   1.075   -6.5%    0.080
2006   1.097   10.3%   1.123    4.5%    0.026
2007   1.043   -4.9%   0.993  -11.6%   -0.050
2008   1.008   -3.4%   1.001    0.8%   -0.007
2009   0.958   -4.9%   1.128   12.7%    0.170