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December 6, 2007

Schrodinger's Bat

Defense and Alphabet Soup

by Dan Fox

"Defense to me is the key to playing baseball."
--Willie Mays

Accurate evaluation of defense remains one of the holy grails of performance analysis, mostly because, from a historical perspective, the way defense has been measured has been stagnant; the same six official fielding stats (games played, total chances, putouts, assists, errors, and fielding percentage) adopted by the National League in 1876 are the same six in use more than 100 years later.

Fortunately, baseball is a game where its structure naturally allows for more rigorous defensive quantification, even if that nascent characteristic was not fully exploited in the past. By assigning players areas of responsibility on the diamond through the codification of positions, each fielder is accountable within his domain when the ball enters it. A good defensive player is then one who is successfully executing plays within his area, or even expanding that area.

In the past quarter of a century, analysts have begun to take advantage of those simple ideas, leading to the development of Bill James' Range Factor (RF), Pete Palmer's Fielding Runs (FR), our own Clay Davenport's Translations (DTs, which include FRAA and FRAR), Mitchell Lichtman's Ultimate Zone Rating (UZR), ESPN's Zone Rating (ZR), David Pinto's Probabilistic Model of Range (PMR), Michael Humphrey's Defensive Regression Analysis (DRA), Shane Jensen's Spatial Aggregate Fielding Evaluation (SAFE), Baseball Info Solution's Plus/Minus system, and even Tom Tango's Scouting Report for the Fans.

With that cornucopia of acronyms to choose from, the discussion within the analytical community has shifted from whether and how defense can be more accurately quantified, to the strengths and weaknesses of the various systems, including combining the metrics to create an über-fielding statistic that incorporates the strengths of various metrics. This week--spurred on by some thinking about the Twins/Rays trade involving Delmon Young--we'll make a small contribution to the ever-growing body of work on the subject of defense.