Analysts have made tremendous progress in the effort to quantify individual defensive performance. Since there’s often noise in the data, a gap still exists between the accuracy of advanced defensive and offensive metrics. As technology improves, though, the aforementioned gap will continue to shrink.

Due to the progress made, defense became perhaps the most noteworthy marketplace inefficiency since on-base percentage. The “defense is the new Moneyball” mantra was a common theme in many articles written about the industry change this past offseason (a Google search with those five words generates over 48,000 results). Several teams opted to open up spots for players who are known more for their glove work than offensive production; one example is the Texas Rangers‘ decision to name rookie Elvis Andrus the starter at shortstop, despite his inexperience and offensive limitations.

Eventually, the market will correct itself (if it hasn’t already), but one economical way that franchises have tried to add a few games to the win column recently has been through improving team defense.

Precisely identifying the number of runs a strong defensive squad will eliminate for a team is still nearly impossible, but organizations are getting much better at coming up with accurate estimates. Evaluating team defense, though, has always been a bit easier than judging individual fielders. Bill James created one of the better metrics to judge team defense decades ago. In his 1978 Baseball Abstract, James introduced Defensive Efficiency Ratio (DER), calling it “simply the best team defensive statistic there is.”

Unlike most numbers featured on the back of a baseball card, DER has withstood the test of time with analysts. DER is fairly basic and doesn’t require a degree in mathematics to understand; it simply shows the rate at which batted balls hit into play (excluding home runs) are turned into outs. The frequency at which a defense can convert balls into outs, and not the numbers of errors made, is an effective way to gauge the actual talent level of a team defense; fielding percentage cannot account for balls that players can’t reach. In other words, DER is the inverse of batting average on balls in play (BABIP) for hitters but on a team level.

DER has certain flaws, of course. It doesn’t account for two outside influences, park and pitching factors, which skew the total. Much like hitting or pitching performance, defensive output will either be enhanced or suppressed by a player’s home ballpark. Coors Field, for example, has historically been a difficult place for teams to play defense. Due to a spacious outfield, the Colorado Rockies‘ DER is impacted by their home field.

Pitching staffs that induce above-average fly ball or ground ball percentages impact the rate of outs produced on balls hit into play. Much of this depends upon the correlation of the pitching staff to the fielders supporting it; for instance, a team with three ball hawks effectively patrolling the outfield gaps will produce a higher DER if its pitching staff predominantly features fly ball pitchers, and vice versa.

Former Baseball Prospectus writer James Click successfully improved upon DER by removing park and pitching factors in a series of articles back in 2003. Click developed Park Adjusted Defensive Efficiency (PADE) and Pitching Independent Defensive Efficiency (PIDE), eliminating forces beyond a defense’s control. A main take-away from his findings is understanding the importance of correctly assessing the make-up of a team’s pitching staff and home environment before making any major roster changes for defensive purposes.

Click, currently a baseball operations coordinator with the Tampa Bay Rays, saw the theory put into practice in 2008. Tampa Bay pulled off one of the most impressive defensive turnarounds ever, going from worst to first in DER.

The then-Devil Rays were atrocious in the field in 2007, posting a .656 DER; for those scoring at home, that marked the lowest total since data became available back in 1954. Brendan Harris provided some offensive pop at shortstop but was miscast at the position. Harris split innings there with Josh Wilson and Ben Zobrist, which had devastating effects on a historically bad defensive team.

Elsewhere in the infield, Japanese import Akinori Iwamura didn’t live up to his Gold Glove reputation (he won six in Japan), providing only average defense at third base. The duo of B.J. Upton and Ty Wigginton at the keystone made for some painful moments as well. Upton moved around the diamond before eventually settling in center field. Carl Crawford again was one of the premier defensive left fielders in baseball. Teammate Delmon Young, however, graded out statistically as one of the worst-fielding outfielders in the league. Unfortunately for Tampa Bay, injuries created a situation in which Young was forced to log a considerable amount of innings in center.

The Rays made it a priority to improve at run prevention in the ensuing offseason, leading to the blockbuster six-player trade with the Minnesota Twins. Matt Garza was the key return chip, but the addition of a solid defensive shortstop in Jason Bartlett played a major factor in Tampa Bay’s decision to pull the trigger.

Replacing the Harris/Wilson/Zobrist trio with a solid defender in Bartlett was indeed a huge addition. Despite the worst statistical season of his career at shortstop, due in part to a knee injury that limited his range, the newcomer helped solidify the middle of the infield.

The Rays’ infield play also vastly improved at other spots. One major factor was the arrival of top prospect Evan Longoria, who quickly established himself as an elite defensive third baseman. Longoria represented a clear upgrade over Iwamura, who became more of an asset when he moved to second base. Carlos Pena, meanwhile, continued to play an above-average first base.

The outfield defense, arguably the best in the game, was also an area of strength. Upton flourished in his new home, combining with Crawford to convert more outs on balls hit into the gaps. Early-season trade acquisition Gabe Gross and rookie Fernando Perez also provided plus defense.

Thanks to the improved defensive alignment, the Rays amazingly posted a major-league-leading .710 DER during the ’08 regular season. The 54-point jump from ’07 was the largest one-year improvement in history. Simply put one out of 20 balls that would have been hits in ’07 turned into outs.

Bill James once wrote that “a great deal of what is perceived as being pitching is in fact defense.” The Rays proved that, as many of the same pitchers from ’07 suddenly watched their BABIPs and ERAs drop considerably; returning pitchers produced a 42-point drop in BABIP. Although the team Fielding Independent Pitching also went down 0.52 points (indicating superior pitching and/or luck on home runs and fly balls; 11.1 HR/FB% in ’07 dropped to 9.3 in ’08), the staff ERA fell 1.71 points as its ERA+ increased from 82 to 116.

After watching the Rays (and Rockies before them) improve in the standings with their enhanced defensive play, several teams took a similar approach this offseason. The sample size is small, but one team out of the group (the Rangers being another great example) that has made a notable improvement in DER is the Cincinnati Reds.

According to Sam Grossman, Manager of Baseball Research and Analysis for the Reds, improving team defense was a priority. “We wanted our defense to turn more balls into outs,” Grossman said in a phone call. “Defensive efficiency is a relatively simple stat but essentially measures exactly that. Our improvement on this front shows that we made a concerted effort to improve on defense, and we certainly hope it continues.”

The Reds finished 29th out of 30 teams with .673 DER in ’08. Boosted by the addition of free agent center fielder Willy Taveras and exceptional right field play of Jay Bruce, Cincinnati ranks sixth in the majors with a .709 DER through Tuesday, May 19.

No longer forced to hide Adam Dunn and Ken Griffey Jr. on the corners, the club now features four players who could play center field if needed; Chris Dickerson and Jerry Hairston are also capable of providing above-average outfield defense. For a team that plays in a small ballpark with a pitching staff consisting of fly ball pitchers, upgrading the outfield defense was crucial.

Some teams have taken the concept to an extreme, focusing too much on run prevention at the expense of run scoring; the Oakland Athletics, the first team to exploit the perceived inefficiency, are a good recent example. Every front office must balance how many runs allowed and scored they expect their team to produce, of course, with trade offs necessary on each side. Still, upgrading team defense can go a long way.

Even in its simplest form, DER does a nice job of determining which teams have strong defenses. Although the tool has been accessible for over 30 years, front offices have only recently exploited this relatively cheap way of increasing win totals. It’s unlikely the recent coverage given to team defense will trigger a long-term change in the way rosters are constructed. With the latest examples of teams bettering their records by improving in DER and advancements in quantifying individual fielding, though, the trend should continue as long as upgrading defensively remains relatively cheap compared to hitting and pitching.