Defense is a slippery beast to get a handle upon. As my Baseball Prospectus colleague Colin Wyers points out, the current generation of defensive metrics — such as Ultimate Zone Rating, Plus/Minus, and BP’s own Fielding Runs Above Average — aren’t as objective as we’d like to believe due to questions about the quality of the data and systematic errors in the way it’s recorded.

That’s why I generally prefer to focus on defense at the team level, using the objective Defensive Efficiency metric as my starting point. DE is simply a record of how often a team turns a ball in play into an out, using the formula 1 – ((H + ROE – HR) / (PA – BB – SO – HBP – HR)). Over the past few years I’ve gotten into the habit of tracking teams’ year-to-year changes in that department, with the 2008 Rays, who set a record by improving 54 points over the previous year, the best-case scenario. The acquisition of shortstop Jason Bartlett, promotion of Evan Longoria and shifts of Akinori Iwamura and B.J. Upton did wonders for the team’s ability to prevent hits on balls in play, giving their young pitching staff an instant boost. I followed that story at BP, and also took note of the Rangers attempting to do something similar last year via the promotion of rookie shortstop Elvis Andrus. The Rangers didn’t win a pennant, but they posted their first winning season since 2004 while staying in contention well into September, and they’ve held onto those gains this year.

In today’s piece, I took a look at which teams have improved the most over last year in that department. While there’s no team which can lay claim to being “this year’s Rays,” the A’s have upgraded their defense at a level which would rank among the top 20 improvements of the Retrosheet era (1954 onward).  The number two team? Well, they’re not exactly underdogs:

The Yankees lead the majors in Defensive Efficiency, converting batted balls into outs at a more efficient clip than any team in the league for the first time since 1998 and any pinstriped team since 1978, though many a club from the Fifties, Sixties and Seventies bettered that mark, because DEs of that era were much higher. Only three teams since 2000 have topped that .718 clip. Here’s what I had to say about the Yanks and the Red Sox:

The Yanks, who haven’t exactly been known for their D in recent years, lead the majors overall; only three teams have posted marks higher than their .718 since the turn of the millennium, the 2001 and 2003 Mariners (.727 and .721) and the 2002 Angels (.719), so it’s probably fair to expect some second-half regression. The Yanks have improved by 21 points since last year and a total of 36 points since 2008 thanks to the arrival of Mark Teixeira, the steady gains of Robinson Cano (+19 FRAA this year), and the swap of Johnny Damon for Curtis Granderson, with Brett Gardner shifting from center to left. The Red Sox, whose general manager Theo Epstein took a lot of heat for his offseason pledge to emphasize run prevention, have seen their defense improve by 20 points despite an injury stack which has allowed Jacoby Ellsbury and Mike Cameron to pair up in left and center just five times, none since April 11. That said, at least in the eyes of FRAA, the upgrade on the left side of the infield from Mike Lowell (-2) and a cast of six shorstops (-3) to Adrian Beltre (+5) and Marco Scutaro (-7) hasn’t been quite the slam-dunk improvement one would have expected.

At the other end of the scale, the team which has declined most, are the Dodgers:

Having accentuated the positive, we’ll move onto lambasting the negative, since eliminating it doesn’t seem to be an option, or even very much fun. And number one on the list of teams that deserve it are the Dodgers, who went from leading the league in DE last year by a whole seven points to ranking 10th this year. Not surprisingly, one key culprit appears to be the loss of Orlando Hudson (+17), though Blake DeWitt and friends have been a respectable two runs above average at the keystone. At third base, Casey Blake has declined (+13 to -5), and Rafael Furcal has dropped off (+13 to +4), surprising given how much more Furcal-like he’s been when available. In the outfield, Matt Kemp has lost 10 runs himself (+8 to -2), a particularly rough blow when coupled with his 20-point drop in True Average. Luckily for the Dodgers, they’re second in the league in strikeout rate, minimizing the number of balls in play.

So while there are no “this year’s Rays,” count the Padres and the Braves among teams whose defensive improvements have helped put them into contention. And count the Dodgers as a team whose decline could cost them a spot in the postseason. If only there was some way they could have kept Orlando Hudson…