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Defensive average

by Sherri Nichols (snichols@rahul.net)



Simply, defensive average is the number of balls fielded by a player at a
position divided by the number of balls hit to that fielder's zone of
responsibility while he's playing that position. It's similar in concept
to STATS' Zone Rating, though the zones for DA seem to be quite a bit
larger than STATS. The zones cover the entire field, and overlap one
another. DA is based on the Baseball Workshop data, which contains
location data for batted balls; these locations are recorded to a finer
degree of granularity than is used in determining DA.


The location of a batted ball is determined by where it is caught or it
lands if it's a fly ball, or where it is either fielded or goes through the
infield for a ground ball. The Project Scoresheet scoring system, which is
used by the Baseball Workshop, includes a diagram of the field, with the
field divided into sections. Multiple sections are lumped together to form
zones of responsibility. A fielder is charged with an opportunity if a
ball is hit into his zone of responsibility, unless the ball is converted
into an out by another player. A fielder is credited with an out if
he converts the ball into an out. Unlike STATS' Zone Rating, a fielder is
only credited with one out when he initiates a double play.


Defensive Average does have some shortcomings. One is that there are a
fair number of easy fly balls in the outfield that could be caught without
difficulty by more than one outfielder. We don't know if there are biases
in the outfield data concerning this; that is, we don't know if on some
teams, the center fielder just takes all of those, inflating his DA, while
on other teams, those are spread more evenly.


There's also the issue of the relationship between the pitching staff and
the defense. Will a bad pitching staff make a good fielder look bad? How
do you tell if the defense is bad because the pitching staff is bad, or if
the pitching staff is bad because the defense is good? 

Additional areas of future research include park effects and age effects.

Legend for DA reports:

Name obvious
G games played at that position
GOps ground-ball opportunities; the number of ground balls hit
to the area of responsibility for that position when this
player was playing that position.
AGOps An adjustment to the GOps to account for batted balls for
which I have no location data.
GO ground balls turned into outs by this player.
AGDA GO/AGOps
FOps fly ball opportunities (anything that's not a ground ball) 
AFOps adjusted fly ball opportunities
FO fly balls converted into outs
AFDA FO/AFOps
FE Fielding errors
TE Throwing errors
1B number of singles through the area of responsibility.
(on ground-balls for infielders, fly-balls for outfielders) 
2B number of doubles through the area of responsibility.
AGO/2B AGOps/2B
AFO/2B AFOps/2b
3B number of triples through the area of responsibility.
DPOps number of double play opportunities, defined as runner on
at least first, less than two out, ground ball to area of
responsibility. 
DPI number of double plays successfully initiated on these
opportunities 
DP% DPI/DPOps

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