Measuring Defensive Metrics

I don't want to use this as another debate about how good or bad Jacoby Ellsbury is or anything, but this is a chance to look at the different metrics and where they come from. I'm going to center on four major metrics and what they attempt to measure as best we know. First up is John Dewan's plus/minus measurement from The Fielding Bible. Only leader boards are available for free and the rest is in the yearly Fielding Bible. This has to be one of the most involved as each players ranking involves video scouts watching every play a player makes and grading him against his peers. The resulting plus or minus value is based on how many more or less plays he make than the rest at that position. This system has a less direct effect on scoring, but how to compare players defensively. Taking a look at 2008 you have Adrian Beltre as the best third basemen in baseball with a +32. On the other end you have Edwin Encarnacion who was a -21. This number is not a run value as I understand it though and more of a comparison tool. It intends to say that Beltre made 53 more plays defensively than Encarnacion in 2008. The plus/minus system plays into another Dewan system called DRS or Defensive Runs Saved. It takes the the plays that added or subtracted to their plus/minus and assign run values to them. This gain or loss of run values results in a total value based on expected runs. Let's see the explanation straight from John: