Breaking Down Adam Dunn’s 2010 – Progress or Regression?

Adam Dunn had a pretty good season this year: he had the second most homers (38), the 12th highest wOBA in the National League, and improved greatly on his horrid defensive performance from 2009. Yet, all is not right in Dunn Land. Seeking a big payoff in free agency, it seems as though Adam Dunn spent 2010 actively trying to inflate his numbers.

Anyone who has studied baseball statistics is aware that the main task of sabermetrics is to describe a player’s skill in a meaningful way. A stat such as batting average, for example, does not capture a player’s offensive abilities all that well since getting a hit depends on many factors that have nothing to do with the hitter’s abilities, such as the defense of the opposing team and the batter’s speed. As a result, a player’s batting average tends to fluctuate fairly wildly throughout a season and sometimes even from season to season.

A more reliable measure would be something like the percentage of pitches a batter swings at since a batter is almost fully in control of whether he gets the bat off of his shoulder (though a batter isn’t going to swing if every pitch is consistently in the dirt or something like that). In fact, swing % does not fluctuate very much and can be said to converge rather quickly to a player’s actually ability level. (For a discussion that is relevant to this topic, look here.

There are several other stats whose fluctuations from year to year or during the season can be reasonably explained by changes in the player’s approach; those stats include percentage of pitches outside of the zone that a player swings at, home run rate, walk percentage, strike percentage, ground ball percentage, and line drive percentage (and a few others I’m sure). Many of them changed drastically for Adam Dunn this season as compared to 2009.

The most shocking change came in Dunn’s walk percentage, which fell by over 30%. This likely came from his increased tendency to swing: Dunn swung at pitches outside of the zone about 47% (!) more often and swung in general about 11% more often this year; both of those statistics correlate fairly strongly with a reduced walk rate. It seems as though Dunn was actively trying to hack away last year.

It seems Dunn may have been hacking with a new swing too. His line drive rate fell by about 13% while his ground ball rate grew by almost 7% and his fly ball rate went up 1%. Most telling, however, is the just over 14% decrease in Dunn’s infield fly ball rate this season. If I were a betting man, I would bet that Dunn spent 2010 swinging for the fences, and without much success either-his HR/AB actually decreased (though only very slightly).

But what did these drastic changes actually get him? Very little in terms of offense-Dunn’s OBP and wOBA actually decreased this year-but a fair bit in notoriety as a bunch of people noticed the high home run rate and batting average he had at midseason. But how different would have Dunn’s numbers been had he not changed his approach at the plate so dramatically? I would guess that his numbers would have been improved if anything, especially his walk and strikeout numbers. That is, Dunn’s new approach seems to have netted him a negative.

Finally, there is the issue of that pesky thing called batting average that was addressed earlier. Dunn’s average the last two seasons seems far too high, especially given his batting averages on balls in play of .324 and .329 the past two years. As is consistent with the Old Player Skills theory (though perhaps more so with the regression toward the mean theory), Dunn’s average should be declining soon, destroying those superficially attractive numbers he has maintained over the course of the past couple years. How quickly the rest of his skills decline will remain to be seen but the real question is whether the Nationals will be willing to gamble that Dunn will remain a top first baseman for the next three years. If they do decide to be risk takers, I would hope that at the very least the Nationals convince Dunn to switch back to his previous swinging habits-at least then he was walking a lot, something he should hopefully be able to do well into old age.