Measuring defense has been a problem that has plagued baseball for years. Fielding percentage is inadequate because you can't be charged for an error on a ball you can't reach. In the modern game, differences in range between two players outweigh differences in sure-handedness. Indeed, Branch Rickey struggled with the question of how to measure effective defense:
"Fielding averages? Utterly worthless as a yardstick. They are not only misleading but deceiving. Take Zeke Bonura, the old White Sox first baseman, generally regarded as a poor fielder. The fielding averages showed that he led the Americn League in fielding for three years. Why? Zeke has "good hands"! Anything he reached, he held. Result: an absence of errors. But he was also slow moving and did not cover much territory. Balls that a quicker man may have fielded went for base hits, but the fielding averages do not reflect this.
Fielding then cannot be measured [...]"
-- Branch Rickey, quoted in LIFE magazine, August 2, 1954
Fortunately, we have made some progress since Rickey penned those words. Two different organizations, STATS, Inc. and Project Scoresheet, have tracked the location of every batted ball in every major league game over the past several years. Each notes the location on a grid spanning the entire field. By assigning positions to be responsible for a certain grid areas of the field, we can measure a fielder's effectiveness on balls hit into his area, independent of his own range.
An image of DA zones (1280x1024, 32K in size) -- Project Scoresheet has been tracking the location of every batted ball in every game for the past several years, using a chart like this one. "Defensive Average", or DA, is the rate at which fielders turn balls hit into their zones into outs. DA is analagous to a fielder's "batting average against" in that it measures times reached base (by hit or error) per opportunity (ball hit into his zone). The areas of responsibility for DA span the entire field -- no portion of the field in considered to be beyond the reach of some fielder. An explanation of Defensive Average can be found here
An image of ZR zones (1280x1024, 28K in size)-- STATS, Inc., publishers of the several popular annual baseball guides, devised their own system of zones to track locations of batted balls. STATS uses this data to measure a fielder's "Zone Rating", or ZR. Zone Rating differs from DA in several ways, notably in that an outfielder is credited with two outs on a double play (DA counts just the fact that a batted ball was converted into an out, without attempting to measure the value of the 2nd out). The areas of responsibility for ZR are smaller for each fielder than in DA. ZR areas of responsibility do not span the entire field -- some areas (for example, deep in the gap between CF and RF) are considered to be a "no man's land" that is ordinarily beyond the reach of fielders, and thus a ball hit there is not considered an opportunity.
Zone Ratings can be found in the annual Player Profiles and Baseball Scoreboard guides published by STATS (available in most bookstores or orderable by calling 800-63-STATS). Defensive Averages are compiled by Sherri Nichols, and are available on the Internet at ftp://ftp.baseball.org/pub/baseball/stats/DA/ and in the Baseball Prospectus.
NEW! Field General Or Backstop: Evaluating the Catcher's Influence on Pitcher Performance Keith Woolner's article from the 1999 Baseball Prospectus that takes an in-depth comprehensive look at catcher's game calling, the magnitude of the effect, and whether it exists as a persistent skill across catchers.
NEW! Catching Up With The General: A Postscript A smaller followup study to the original Field General Or Backstop article, looking further into catcher's defense.
