WE'VE COME A long way in measuring defense in the last 20 years, moving from assists, putouts and errors to tools that get us closer to the goal: gauging defensive performance in the context of opportunity. We don't judge hitters merely by counting their strikeouts or hits or home runs; we want to know how often those things occur in relation to how often they come to the plate. So we count at bats and plate appearances and calculate batting average and OBP, which tell us more than the raw numbers.
Defensive statistics do the same thing by putting all defenders' performances in the context of their opportunity to make plays. This resolves the biggest problem with fielding percentage, which is that an error by a shortstop and a single past the shortstop—the same thing to the team, a baserunner added with no out recorded—are judged differently. A formula-based system, such as Ultimate Zone Rating (Baltimore third baseman Manny Machado,above, leads all defenders this season), uses batted-ball and play-by-play data to calculate how many runs a player saves or costs his team relative to an average defender given the same set of opportunities. An observational system like Defensive Runs Saved goes a step further, assigning pluses and minuses to everything a player can do in the field—things like playing a ball off a wall, handling a tough throw or blocking the plate. The organizing principle is that every ball off the bat, every runner taking a base, is an opportunity for a fielder to make a play.
Because of their newness, defensive statistics are viewed with skepticism. Neither UZR nor runs saved is gospel, but the two together provide an empirical basis for forming an opinion about a player's defensive performance. Not his skills, not his style, but his performance. We don't evaluate hitters by how they look at the plate, and we shouldn't evaluate fielders by how they look in the field. Style can be overly influential in our evaluations of defense; the substance is turning balls into outs. The tools we have now are better than observation and far better than the traditional statistics. They're the best way to determine how good a player is in the field.