It might be the biggest debate in baseball, statistically speaking. We’re well past RBI and pitcher wins, by now. WAR is a big debate, but not so much because of the offensive statistics, or the baserunning figures. WAR is debated largely due to the thing I had in mind when I wrote that first sentence, the one about the biggest debate in baseball, statistically speaking: defense.

There’s still a strong “eye test” contingent. Folks who believe you just can’t put a number on defense. On the other side, there’s a staunch numbers crowd. The crowd that argues, well, you can’t see every play from every defender, and you also can’t ignore or probably even be aware of your own internal biases; I’ll stick with the numbers. Where it gets real tricky is that, even within the numbers-oriented crowd, there’s some skepticism of those very numbers. There’s some concerns with the methodology. Defensive shifts make things extra tough.

So for the most part, we shrug our shoulders and accept that, for as far as these things have come over the years, we’ve still got to do some leg work. If we really want to gain an idea of a player’s defensive ability, we’ve got to just take it all in, and look for clues along the way. What does each defensive metric say? When they agree on one thing or another, we’ve got ourselves a clue. How about errors? They’re not the best, but they’re not worthless. Do they line up with what we saw in the advanced stats? Clue. Check out some spray charts, or Inside Edge. Watch some film, and read some scouting reports. Plenty of clues to be found in there, especially given all you’ve learned along the way. Do all this, and you’ll have a pretty good idea. Even if one number or one play or one quote goes against what you’ve concluded, that’s the point; your body of research holds more weight than that one thing that purports to invalidate your findings.

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Each year, Tom Tango does a fun little project called the Fans Scouting Report. The nature of the project, essentially, is to crowdsource the eye test. There’s plenty of ways to use the data, and I’ve settled on one, for now. I wanted to look for improvement, and I wanted to look for agreement, using both the eye test, and the advanced numbers. I used three sources of defensive metrics (UZR, DRS, FRAA) for fielders with at least 500 innings in 2014, and 2015. I averaged those to get component defensive runs above average figures, and then, I compared against the Fans Scouting Report’s numbers. Using some z-scores, I could come up with an overall ranking of agreed-upon improvement.

Kevin Kiermaier popped up near the top, and how could he not? He just had arguably the best defensive season on record. Both of the Marlins’ middle infielders made the top five, and that’s been covered here, too. But one name stood above the rest. For one player, the eye test crowd and the metrics crowd can come together and anoint a most improved defender, and that defender is Xander Bogaerts.

Last year, by all accounts, Bogaerts was bad. UZR had him three runs below average, in less than a full season’s worth of work. DRS said -9. FRAA had -10. The fans saw a below average defender, as well. But this season, UZR saw a four-run improvement. DRS saw a nine-run improvement, and so did FRAA. And the fans saw a big step forward, too:

Xander Bogaerts, Fans Scouting Report, 2014 vs. 2015 Season Instincts First Step Speed Hands Release Arm Strength Arm Accuracy Overall FSR 2014 42 45 56 36 29 60 30 42 -11 2015 64 61 60 64 62 64 62 62 1

To be fair, there’s some third base mixed in with the 2014 numbers, but position is instructed to be ignored when the votes are cast, and the majority of his playing time came at short anyway.

The fans didn’t see much change in Bogaerts overall speed, or athleticism. But the instincts, the first step, and the glovework all went from below- to above-average. The numbers agree: Bogaerts’ range runs and error runs were major sources of improvement. You imagine he might be positioning himself better. You imagine he’s gaining a better feel for how balls come off the bat at the major league level, and how major league infields play. The glove can always be improved with repetitions, and scouts first saw the glove work developing in 2013.

Red Sox fielding instructor Brian Butterfield said in July that Bogaerts had made a “quantum leap” defensively from 2014 to 2015. When Bogaerts ranged to his right and made a nice play in the hole against Erick Aybar that same month, manager John Farrell said it was “a play he doesn’t make a year ago.”

But the biggest step forward, according to the eye test, was in the arm. The strength was never in question. In all four years of Bogaerts’ scouting reports from BaseballAmerica, dating back to 2010, the phrase “plus arm strength” is invoked. He’s always had the arm strength; it was the range and the glovework that led so many to believe he’d move to third base or right field eventually.

We’ve used the advanced numbers, and the fans’ eye test, and some scouting reports and personnel quotes, but I let’s go one step further and incorporate some visuals to see if there’s anything to learn about Bogaerts’ apparent arm improvement. I picked two sets of double plays at random, three from 2014, and three from 2015, and compared, paying attention to Bogaerts:

The thing that first sticks out is that Bogaerts has made an adjustment to his throwing mechanics, going to a lower arm slot. In 2014, Bogaerts was getting more on top of the ball, and in that top-left frame, he spiked it in the dirt. Could be a habit that carried over from his time spent at third base, and when he got back to playing everyday shortstop, he settled back into the groove of his typical middle infield mechanics.

This is specific only to double plays, but the other thing that stuck out, and the Red Sox broadcast crew spoke to this during one of the 2015 clips, is Bogaerts’ footwork around the bag, namely his proximity to it. The TV crew noted that in 2014, Bogaerts had a tendency to take his body too far toward right field, forcing himself to throw across his body to first. In the bottom-left frame, Bogaerts nearly threw the ball away and into the dugout, but was saved by a nice stretch from Mike Napoli. But note Bogaerts’ proximity to the bag in the 2015 clips. He’s taking a more direct route to the base, and staying tighter to the bag with his throws, leaving his body more square and putting himself in a better position to make accurate throws.

It should be noted that, by any of these measures, Bogaerts was not a plus defender at shortstop. DRS had him exactly league-average with UZR and the fans a run above and FRAA a run below. But, by all measures, Bogaerts held his own at shortstop last year, and that’s a massive upgrade from where he was a year ago. A year ago, he was unplayable, and a move to a lesser position looked like it might happen sooner than folks thought. Now, Bogaerts looks to have bought himself some time. Scouts noticed the footwork, the range, and the glove making strides in 2013, and by all accounts, they made even bigger strides from 2014 to 2015. Bogaerts looks to have cleaned up his throwing mechanics, allowing himself to better tap into the plus arm strength he’s always had with more accurate throws.

Bogaerts has brought himself up to being a league-average defender at short, just a year after being a disaster in the field. And with his bat, and his age, league-average defense at baseball’s toughest position is all Bogaerts needs to be one of the game’s most valuable players.