The current state of defensive metrics is “pretty good”, in evaluating who the best defenders are. There are mixed views on that, but largely the metrics used to generate the SABR Defensive Index (SDI) are aligned with Statcast’s OAA (to date) by player. Baseball Info Solutions’ DRS (for most of this blog, I will just reference DRS, since UZR uses the same data) and RED, based on STATS’ ZR data, both do a good job of describing turning batted balls into out.
From the information Tom Tango talks about here, we will be able to understand *why* players are better or worse at turning batted balls into outs. Which players get the best jumps. Which players take the best route. What is the spread of those talents and how predictable is it.
What I have seen (though not fully studied) is players come into the league and learn to adjust to the speed of the game, performing around or below average. In their second, third, fourth seasons, they use their youth – speed, reflexes, reaction – to maximize the balls they turn into outs, peaking in their runs saved early, and then begin the decline into designated hitter, or just retirement.
The defensive aging curve is not dissimilar to the offensive aging curve with respect to shape. However, it is a couple of years earlier. Defense is a younger man’s game because much of it relies on speed and reaction. As more data rolls out of Statcast at baseball savant (nee Daren Willman’s site), we’ll be better at understanding how a player succeeds, and probably better predicting whether he will continue to do so.