David Pinto at Baseballmusings.com has developed a defensive measurement system called Pobabilistic Model of Range (PMR).
Baseballmusings.com has begun crunching numbers for the 2008 version of the model. PMR is one of the range based defensive measures which uses play by play data. The PMR methodology has several features which are different from most zone based methods, and potentially improve its accuracy. For instance, PMR does a better job of controlling for ballpark effects. PMR also divides the baseball field into vertical slices rather than zones, and uses probabilistic concepts to determine whether fielders beat the expected value for making particular plays.
As explained by Pinto:
Note that a team can post a poor DER during the season, but do well in this model if the balls put into play were extremely difficult to field.
PMR ranks the Astros as the 6th best defensive team in 2008. The only teams which are better, according to PMR: Blue Jays, Braves, Rays, A's, Red Sox. (Interestingly, all but the Braves among those teams are viewed as having sabermetric GMs.) The ranking basically confirms the Hardball Times' team Revised Zone Rating (RZR) rankings which have the Astros rated as tied with the Cards for best defensive teams in the NL (for the NL only, the Astros are No. 2 according to PMR). So, it's safe to conclude that the Astros had a good defensive season in 2008.
How much of an improvement for the Astros in 2008? In 2007, the Astros were ranked 25th among major league teams by PMR.
I have attempted to follow the Astros' players' defensive performance over the 2008 season. However, I have been waiting for the PMR results before I try to summarize the Astros' defensive results for the season. Normally it takes a few days for Pinto to crank out results for all of the player positions. When he has published all of those results, I intend on writing up a summary comparison of various defensive measures for individual Astros' players.