Those who think he is a terrible outfielder because of dubious fielding metrics (that, imo, are often wildly inacurate) are just wrong. Nobody gives up astronomical numbers of runs in LF -- 30-40+ runs a season. It's silly. There aren't enough at-the-margin plays out there to make that much difference. How can you possibly watch the game for years, have a sense of the range of an "average" outfielder, and think that Dunn (or anybody else) has that much negative impact?Boz is referring to stats like Ultimate Zone Rating, Probabilistic Model of Range, John Dewan's plus/minus system, and Baseball Prospectus's Runs Above Average. I keep permanent links to most of these stats on the right hand bar of this page for reference. These stats are imperfect and have a larger margin for error than stats like OBP or SLG. But these stats have proven to have real predictive value, especially given adequate sample sizes.
Why is it so hard to believe that a left fielder can cost his team 30-40 runs? Let's look at some more familiar stats to establish a ballpark range of what's reasonable.
An outfielder can prevent runs basically three ways: range, throwing arm, and errors. Errors are the most familiar source of "lost runs," so let's start there. The left fielders league-wide in 2008 made 162 errors for an average of 5.4 per team. Detroit, the team with the most errors in LF in 2008, committed twelve. Milwaukee was the best with zero.
Tom Tango has calculated the run value of each batting event, and the average error costs .506 runs, assuming a five-run-per-game scoring environment. MLB is at just 4.64 runs per game this year, so the current run value of an error is probably closer to .475 run.
Then again, errors by left fielders tend to be more costly than errors by infielders (for obvious reasons). For instance, the run value of a double, the de facto outcome of many outfield errors, is .781 run. Let's throw a reasonable dart and call it 0.575 run per LF error. That would mean the difference between the most and fewest errors among left fielders (12 * 0.575) was worth about seven runs in 2008.
Now let's look at throwing arms. League-wide, there were 283 assists by left fielders, or 9.4 per team. The league-leader was San Francisco with 16, and the teams with the fewest were San Diego, Tampa Bay, and Pittsburgh with five.
Tango doesn't list the run value of an outfield assist, but the value of an out is .304. Outfield assists tend to be particularly valuable outs, however, so let's round up to .4. That means that the range from best to worst LF arms is about 4.5 runs.
Outfield assists are just part of the story when it comes to preventing runs with throws. Strong arms and good throws also prevent runners from even trying to advance.
The only way to track these events is to look at the play-by-play data, and The Hardball Times has done just that. By their count, almost exactly the same number of runs were prevented by holding runners as throwing them out among qualified LFs in 2008 (1510 "kills" vs. 1517 "holds"). Assuming they're right, let's double the value of LF arms to nine.
Finally, we come to range, where the biggest difference between good and bad fielders exists.
In 2008, Kansas City led baseball in LF put-outs with 368. Philly and Toronto tied for the fewest put-outs at 278. Therefore, assuming 0.575 runs per out, the gap between the best and worst teams in LF range is worth as much as 90 outs and 52 runs per season.
These rankings make sense, since David DeJesus, an excellent fielder, got the majority of the innings in LF for KC that year, while Pat Burrell and Adam Lind, neither good fielders, played the most innings for Philly and Toronto.
Of course, put-outs are also affected by the flyball rate of the pitching staff. Indeed, KC pitchers had among the higher flyball rates in the league at 38.6%. As a result, Royals outfielders saw 1573 flyballs all season, and DeJesus and the other left fielders converted 23.4% of them into outs.
Meanwhile, Toronto and Philly were towards the bottom in flyball rate at 33.5% and 33.8%, respectively. Total, Philly's OFs saw 1360 flyballs and Toronto's saw 1345. Philly LFs converted 20.4% of outfield flyballs into outs and Toronto LFs converted 20.7%.
We also know that the average team made 307 put-outs in left in 2008 in 1428 outfield flyballs with the average LF converting 21.5% into outs.
All this shows is that, yes, David DeJesus and friends got more chances than average, and Pat Burrell and Adam Lind had fewer. But even if the Phillies, Blue Jays, and Royals all had average numbers of opportunities, the Royals still would have gotten about 43 more outs from their superior range.
So let's assume then that over the course of the season, the gap from the best and worst range among left fielders is 43 outs. Further, let's assume that the run value of these additional outs is the same as an outfield error, which we pegged conservatively at .575. (This is almost surely too low, since the majority of difficult plays made in the outfield prevent extra-base hits.) That gives us a gap of about 25 range runs.
Adding together the run values of arms, range, and errors, we get a total of 41 runs. So it's certainly not, as Boz says, "silly" to think a single left fielder could cost his team 30-40 runs on defense. It seems pretty typical, in fact. And for a team like the Nationals without a single strikeout pitcher in the rotation, the stakes are even higher.
Whether Dunn is as bad as the very worst is a topic for another post. When I do my own wins above replacement projections, I try to reduce the margin for error by averaging out three years worth of fielding data from at least a couple different systems. Going into this season, using three years of UZR and Rate2, I had Dunn as a 15.71 runs below average left fielder over the course of 150 games.
Regardless, it's a mistake to just dismiss out of hand the large number of runs that a poor fielder--even a poor left fielder--can cost you. And a run allowed in the field is worth exactly the same amount on the scoreboard as a run scored with the bat.
- Update: The weekly UZR update is up at Fangraphs. With the usual caution about small sample size (UZR really isn't all that reliable on a week-by-week basis), but with a grain of salt his numbers are creeping up, as some of the commenters suggested. His UZR hasn't sunk at all from -7.5, meaning he's been average for the last week. His season-long projection (UZR/150) has risen to -28.