A week ago, Jeff Sullivan—an astonishingly good baseball writer—tried to figure out why the Seattle Mariners were the new favorites to win the AL West, a division which the Astros were not only expected to win but to be the only particularly good team in. To do it, he used a statistic I hadn't seen anyone use before: slugging percentage on balls in play.
It's a pretty simple idea. Slugging percentage is sort of like batting average but takes into account how many total bases hitters get on their hits. So slugging average on balls in play is basically a measure of not just how often batters get hits when they hit the ball, but the quality of those hits.
Sullivan used it to see if the Mariners' approach to defense had improved the team's SLGBIP. Sure enough, it seems to have worked. The Mariners, when he took the data, had the third-best change from 2015 to 2016, and the second-best SLGBIP overall. But what struck me were the teams with the best and second-best changes in SLGBIP—because they're the Cubs and the White Sox. I re-ran the data using Baseball Reference's powerful Play Index, and the White Sox have just edged ahead of the Cubs.
Change in Slugging Percentage on Balls in Play, 2015-2016
| White Sox | |
|---|---|
| Cubs | |
| Mariners | |
| Rays | |
| Nationals | |
| Dodgers | |
| Indians | |
| Rangers | |
| Phillies | |
| Reds | |
| Royals | |
| Rockies | |
| Cardinals | |
| Tigers | |
| Orioles | |
| Braves | |
| Blue Jays | |
| Red Sox | |
| Angels | |
| Marlins | |
| Diamondbacks | |
| Padres | |
| Pirates | |
| Athletics | |
| Brewers | |
| Mets | |
| Yankees | |
| Twins | |
| Astros | |
| Giants |
The Cubs were obviously expected to be great this year. Part of the reason they were expected to be great is that the numbers suggested that they improved their defense, as they've done consistently over the past few years. The White Sox are a much bigger surprise. How did they do it?
1. Acquiring Austin Jackson. He's an okay defensive center fielder, with an Ultimate Zone Rating per 150 innings of 1.5 at the position over his career, barely above average. He's actually below average so far this year. But that's better than…
2. Adam Eaton, who moved to right field. Defensive metrics hated Eaton in center last year, and his career UZR/150 at the position is -9.8. In right field—an admittedly small sample size—it's 44.7. South Side Sox went deep into this recently, as did MLB.com. Eaton's move to right meant that last year's right fielder was without a position, so…
3. Avisail Garcia moved to DH. This was something of a risk, as Garcia was a mediocre hitter last year, hitting .257/.309/.365. He's hitting better this year, slashing .265/.333/.442, and getting his slugging percentage over .400 is particularly important. Either way, Garcia moving to DH means he's not playing defense, which he's not especially good at.
And it's those position changes that seem to have made the most difference in the Sox's defense. The personnel changes in the infield have been a wash on the defensive end, if even a bit worse, though the improved hitting from the team's infielders more than makes up for it. In the outfield, the team has gone from a collective -8.2 UZR/150 (second worst in baseball) to a collective 14.9 UZR/150 (fourth-best in baseball, just behind the Cubs). And it all started with the acquisition of one, just-OK defensive player.