Baseball Prospectus has an interesting article - the bulk is restricted to subscribers, but I feel I can pick out some extracts from that, to shed some light on to our fielding woes this season. Defensive Efficiency is a metric that measures the percentage of balls in play which a defense converts into outs. The higher the percentage, the better your defense. However, that raw figure masks variations between parks: as the article says, "The amount of foul territory, size of the outfield, irregularities in the field's dimensions, altitude, and surface of play can all have an impact on the degree of difficulty when it comes to defending one's home turf."

But just as you can come up with park factors that take into account a stadium's impact on hitters, so you can do so for its impact on fielders. Fenway, Dolphin Stadium, Progressive Field (formerly Jacobs Field), and Coors Field are hardest for fielders, while the Metrodome, Tropicana Field, McAfee Coliseum, and Petco Park are easiest. Arizona's three-year factor is 0.9950, slightly tougher than average. You can use this to come up with an overall figure for the percentage above or below ML average that a team converts into outs, when adjusted for the degree of difficulty provided by their home field. Each 1% is worth about 13 additional runs saved or allowed, which would be more than one win in the standings.

Overall, Arizona's defense is ranked ninth in the majors, having converted 0.59% more balls in play into outs than would be expected for a team playing at Chase. For comparison, top are the Cubs, at +2.60%, and worst are the Reds, all the way down at -4.30%. That figure is the best in the division, by quite some distance - we're the only team better than average, well ahead of the Giants (-0.94%); the rest of the West trails in below them, with the Padres and Rockies tied for worst, at -1.23%. So, despite how it may feel at times, our defense has been better than our rivals at doing what it's supposed to: making outs.

However, it is also important to note that our figure is a significant drop on the +1.19% which we had last season - good enough for fifth overall. That does tie with what we've seen this year: a disappointing regression with the glove by many players, despite then being a year more experienced and at the age where they should not yet be expected to be slowing down. It's difficult to break down the source of the drop, but I do note Mark Reynolds has gone from -6 Fielding Runs Above Average last season, down to -11 this year. The replacement of Carlos Quentin (-1) with Justin Upton (-7) in right, is likely also a contributory factor.