Sunday, April 25, 2010

Fielding Bible Stats at FanGraphs

You may have noticed that FanGraphs recently added some new defensive statistics. It's too early in the season to analyze player fielding but I will be using these measures a lot in the future.  So, I'll take some time today to explain them.  Much of this post was abstracted directly from my book Beyond Batting Average which discusses these and many other statistics in great detail.

The newly listed FanGraphs statistics were originally published in the Fielding Bible - Volume II and are based on John Dewan's plus/minus system.  The two statistics most people will be using most often are

rPM - plus/minus runs saved

DRS - defensive runs saved.

The Fielding Bible system is similar to Mitchel Lichtman's Ultimate Zone Rating (UZR) which has been at FanGraphs for a while.  Both systems use detailed play-by-play data on location and type of batted ball from Baseball Info Solutions (BIS).  The methods are similar in theory but use somewhat different algorithms and thus produce varying results for some players.

BIS video scouts watch videotape of every MLB game and every play is entered into a database which includes exact direction, distance, speed and type of every batted ball. Direction and distance are entered by simply recording the exact location of the ball on a replica of the field shown on the computer screen. The speed of a batted ball is recorded as soft, medium and hard.  Each play is also categorized according to batted ball type: groundball, line drive, fly ball, fliner (a cross between a fly ball and a line drive) or bunt.

Dewan first published the plus/minus methodology and results in The Fielding Bible in 2006 and added enhancements in the The Fielding Bible – Volume II in 2009.  The plus/minus system breaks the field into small areas and determines the probabilities of players making plays in each area based on location, speed (hard, medium, soft) and batted ball type (ground ball, fly ball, line drive, bunt).  The system uses these probabilities to determine how many outs each player was expected to make and how many he actually did make in comparison to the average player.

Dewan explains that each type of play (e.g. softly hit ground ball hit into the hole between short and third) has a difficulty determined by how many times all fielders in MLB converted that play into an out.  For example, if that play was made 30% of the time, then a player gets a score of +0.70 for making that play.  If he doesn't make that play, then he gets a -0.30 score.

The scores on all of a player’s plays are summed to compute his +/- rating.  This rating represents how many plays the player made above or below the number of plays an average player at his position would have been expected to make given the same opportunities. For example, Adam Everett had a +/- of +9 in 2009 indicating that he made nine more plays than the average shortstop would have been expected to make given the same opportunities. 

Dewan converts plays made above average to defensive runs above average using situational run expectancies.  Based on these expectancies, Dewan determined that each plus/minus point for a shortstop is worth 0.76 runs.  For example, Everett cost his team 9 x 0.76 = 7 runs in comparison to the average shortstop.  For outfielders and corner infielders, Dewan adjusts the +/- figure for the potential of individual balls going for extra bases. The result is the rPM statistic.

Dewan also introduced defensive runs saved (DRS) in The Fielding Bible – Volume II.  DRS is an extension of the plus/minus system.  In addition to +/- runs saved, DRS includes other sources of runs saved by position:

rSB - stolen base runs cost/saved for catchers

rGDP - Double play runs cost/saved for infielders

rARM - Arm runs cost/saved for outfielders

rHR - home run runs cost/saved for outfielders

For example, Everett had 7 plus/minus runs and -1 double play runs for a total of 6 DRS in 2009. This is similar to his UZR of 5.3.  So DRS serves as a confirmation of UZR in this case.  Some players do not match so closely on DRS and UZR but that is a topic for another day.  For now, I'll say a little more about the four additional statistics listed above.

The rSB measure is based on the number of stolen bases and the success rate against an individual catcher.  However, it also takes into account the stolen bases and success rate against the pitchers they caught.  So, if one pitcher on a staff was bad at holding base runners and one catcher caught a disproportionate number of that pitcher's games (compared to other catchers on the same team), the catcher will not be unduly penalized for something that wasn't his fault.  Gerald Laird saved the Tigers nine more stolen bases above what would have been expected from the average catcher in 2009.  Each stolen base saved is worth .62 runs so Laird had a MLB leading seven rSB.

The rPM statistic looks at how many plays a player made above what an average player would have been expected to make but does not give a player credit for turning double plays.  The Fielding Bible system counts the number of times an infielder had opportunity to be involved in a double play (including fielding balls and making pivots) and how many times he successfully participated in a double play.  Like the other statistics above, rGDP is converted to runs cost/saved.  Adam Everett had a -1 rGDP so he cost the Tigers one run on double play balls in 2009.

The rPM statistic only considers an outfielder's ability to make catches and does not consider the outfielders arm.  The rARM statistic gives an outfielder credit for his ability to throw runners out and to prevent runners from advancing. For example, when a batter hits a single to left field with a runner on first and second base unoccupied, the runner either stops at second or attempts to advance to third.  If the runner stops at second, then the outfielder would get credit for a hold.  If the runner is thrown out at third, then the outfielder is credited with a kill.

For each outfielder, Dewan counts the numbers of advancement opportunities, holds and kills.  He then determines how each outfielder compares to league average in both kills and holds.  Finally, using situational run expectancies (e.g. How likely is it for a run to score with a man on first and third and no outs compared to a runner on first and one out?), he then calculates runs saved above/below average for each outfielder.  Bobby Abreu was the MLB leader with 11 runs saved with his throwing arm in 2009. If we only look at Abreu's rPM of -10, he rates as a poor outfielder.  However, if we add his arm rating to the equation, his final DRS is +1.

BIS also tracks how many times outfielders caught balls that would have been home runs if the ball was not caught.  That is, the ball was heading over the fence and the outfielder either reached over the wall and pulled it back in or caught it just before it would have gone out.  Adam Jones saved the Orioles an MLB best six runs with home run saving catches (rHR) in 2009.

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