The older-timers know the deal, but for newer folks, this data is compiled with a script that runs through every game log and parses out defensive plays made in each ball hit into each zone--then I compare how a team is doing vs. the overall league. In this particular case, I've generalized down to just Ground Balls, Fly Balls, and Line Drives (IF/OF). When I get a chance I'll post more granular stuff, just cause it's interesting, right?
Anyway, I can't get down to individual players with this kind of study, but ultimately, the script calculates the number of plays each team has made relative to the number they would have made if they had a league average defense.
Bottom line: right now, Rockville has the leageu's best defense, having made 28.3 plays more than they would have had they been fielding an average defense. Phoenix is close behind.
BBA DEFENSE: PLAYS ABOVE AVERAGE