I will say up-front, that it is quite possible I just do not know enough about defensive stats.
However, it seems to me that defensive stats are just poor.
The old-standard used for years has been Range Factor.
As I understand RF, it is Putouts + Assists x 9 divided by defensive innings played.
What is accounting for positioning? What about the players that have to ‘cheat’ a bit to help cover the player next to them? What about the players next to them that cover a ton of ground? What if you have pitching staff that either gets a lot of fly balls or a lot of grounders? What about park-effects?
The current most popular defensive statistic is probably Ultimate Zone Rating on FanGraphs.
Is there really value there? In 2008, Nick Markakis had a 12.1 Fielding Runs above Avg based on UZR. In 2009, Markakis has a – 9 Fielding Runs above Avg based on UZR.
Now, Markakis had 3 errors all of 2008, and has 3 errors already in 2009; but obviously that only tells so much.
He had 3 defensive double-plays in 2008 vs 0 defensive double-plays in 2009; but I believe UZR does not include double-plays.
He had 17 outfield assists in 2008, vs 8 so far in 2009.
The main difference appears to be put-outs. In 2008, Markakis had 1,367 defensive innings, and 314 put outs. In 2009, Markakis has 736.2 defensive innings, and 136 put outs.
Fangraphs shows the calculation of UZR as ARM+DPR+RngR+ErrR.
ARM (outfield arm runs): Outfielder’s get credit (plus or minus) depending on what the runners do on a hit or a fly ball out. A runner can stay put, advance, or get thrown out. A fielder will get credit not only if he throws out more than his share of runners, but also if he keeps more than his share of runners from advancing extra bases.
DPR (double play runs): The number of runs above or below average a fielder is, based on the number double plays versus the number forces at second they get, as compared to an average fielder at that position, given the speed and location of the ball and the handedness of the batter.
RngR (range runs): The number of runs above or below average a fielder is, determined by how the fielder is able to get to balls hit in his vicinity.
ErrR (error runs): The number of runs above or below average a fielder is, determined by the number of errors he makes as compared to an average fielder at that position given the same distribution of balls in play.
What value can a defensive statistic have, if in 2008 Markakis had an ARM rating of 7.2, and now in 2009 it is 1.8?
In 2008 his RngR was 4.7, and now it is -10.8?
Are you kidding me? Even if Markakis is not getting to balls on the RF line quite as often as he did last year… the idea that his range has diminished that much in a year is a joke.
Maybe there are less fly balls, and less opportunities?
Maybe there are less runners on the bases overall, and less opportunities to keep a runner put?
Maybe Jones is covering more ground in CF?
Every series, we see the respect Markakis gets… opposing teams do not challenge him.
Balls that do get past him, he usually recovers quickly and makes a strong accurate throw to the cut-off, or the base ahead.
I am glad to listen to feed-back here of why I have this wrong.
I think defensive numbers can be used as a tool, but I think they are rather crude measurements, that provide rather minimal information overall.
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