Saturday, May 5, 2012

Dissecting David DeJesus's Dubious Defensive Data

As I noted in my previous post, David DeJesus took a massive hit with Baseball Reference's new WAR calculation. The difference can be accounted for almost entirely from the metric used for defense. Previously, rWAR used Total Zone (TZ) to evaluate defense for all seasons. It's hard to wrap my head entirely around all the defensive metrics, but as I understand it, TZ is one of the more simplistic, and reflects mostly the quantity of plays made with little or no adjustment for the difficulty of those plays. TZ loves DeJesus's defense with the Royals, rating him 66 runs above average (RAA) in his time in KC (2003-10). That's third best among outfielders in that time frame, behind only Coco Crisp and Ichiro Suzuki.

The new rWAR switched to using Defensive Runs Saved (DRS) starting with the 2003 season. DRS attempts to rate the difficulty of every ball in play and give players credit or discredit based on the plays they do and don't make. DRS paints an entirely different picture of DeJesus's 910 games in the Royals outfield, showing him as basically average at just 3 RAA. This massive discrepancy between TZ and DRS may mean that DeJesus made a lot of plays in the OF, but didn't necessarily make the tough plays. That could be since DeJesus played behind some terrible pitching in those years, staffs that were giving up a lot of contact and a lot of fly balls.

Ultimate Zone Rating (UZR) uses a similar framework as DRS, but paints yet another picture of DeJesus: 31 RAA. Well above average, but not the beast portrayed by TZ. Three different fielding metrics, three completely different assessments of DeJesus's defense despite the large sample size. It's a good reminder of how unreliable fielding metrics are, and therefore how imprecise WAR really is. I've defended DeJesus in the past on this blog based largely on his previous rWAR numbers using those huge TZ numbers. I was called out by a couple of folks for leaning so heavily on WAR, and the discrepancy in the defensive numbers shows they had a point. The framework of WAR is brilliant, but the defensive data going in--no matter which metric--is questionable.

My eyes saw an above average but not elite defensive outfielder. Much like DeJesus's hitting, he seemed reliably good year in and year out in the field. How did you view DeJesus's defense? Vote in the poll on the right, and also weigh in on whether or not you think he belongs in the team Hall of Fame eventually.

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