Saturday, October 23, 2010

An Introduction to Revised Zone Rating Stats

I think that most of the readers of this blog have checked out the fielding statistics at FanGraphs from time to time.  Statistics such as Ultimate Zone Rating (UZR) and Plus/Minus or Defensive Runs Saved (DRS) give us an idea of a player’s overall fielding performance.  They are given as runs saved above what would be expected from the average player at the given position.  They are discussed in more detail in the fielding glossary.

You may have noticed two additional columns at FanGraphs labeled RZR and OOZ.  These are the Revised Zone Rating statistics developed by John Dewan, president of Baseball Info Solutions.  Dewan later developed the more detailed DRS metric.   The Revised Zone Rating system is comprised of the following measures:

  1. Balls in play within a fielder’s zone (BIZ)
  2. Plays made in the zone (Plays)
  3. Proportion of balls in zone converted into outs (RZR)
  4. Plays made outside the zone (OOZ)

A play is considered to be inside a positional zone if half the balls hit into that area are converted into outs by all the players in baseball at that position.  While the Revised Zone Rating system is less sophisticated and less accurate than UZR and DRS, it is useful because it separates the plays a player made inside his zone from the plays outside his zone.

Table 1 below shows how the distinction between in-zone and out-of-zone plays can be useful.  Texas Rangers shortstop Elvis Andrus converted 308 out of 377 (81.7%) of balls in his zone into outs in 2010.  Yunel Escobar who split time with the Braves and Blue Jays had a similar number of balls hit into his zone (367) but had a smaller RZR ( 79.8%).  Conversely, Andrus made fewer out-of-zone plays (46) than Escobar (72).  These data suggest that Andrus may have been the steadier fielder on balls hit in the shortstop zone but Escobar was perhaps better at making difficult plays.  

Table 1: Revised Zone Rating Statistics for Elvis Andrus and Yunel Escobar

Elvis Andrus 377 308 0.817 46
Yunel Escobar 367 293 0.798 72

Table 2 presents some zone rating statisics for the few Tigers who played regularly throughout the year.  We can see, for example, that there 19 center fielders with roughly 100 or more games worth of innings.  Austin Jackson played 1,256 innings and had a revised zone rating of 92.9%.  That ranked him second in the majors behind Denard Span of the Twins.  Jackson also made a league leading 109 out-of-zone plays.  So, based on these measures, Jackson was outstanding on both relatively easy plays and more difficult plays. 

The biggest surprise on this table for me was that Inge ranked near the bottom of the league in makes plays outside his zone.  He did better on in-zone plays.

Table 2: Revised Zone Rating Statistics for Tigers in 2010

Pos Name N Inn RZR Rank OOZ Rank
1B Miguel Cabrera 22 1,285 0.748 15 37 12
3B Brandon Inge 19 1,226 0.733 10 30 17
CF Austin Jackson 19 1,256 0.929 2 109 1

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