From: hbrooks@uiatma.atmos.uiuc.edu (Harold_Brooks)
Subject: Standards for defensive statistics (Was Re: Mattingly...)
Organization: NSSL
Distribution: na
Date: Fri, 6 Nov 1992 17:03:51 GMT

In article <7943@transfer.stratus.com> gene@bogus.sw.stratus.com (Eugene Loparco) writes:
>In article <7941@transfer.stratus.com> jmann@vineland.pubs.stratus.com writes:
>>In article <7936@transfer.stratus.com> gene@bogus.sw.stratus.com (Eugene Loparco
) writes:
>
>Those have been proven wrong (well, one anyway), so they are not opinions,
>they are misconceptions.  The "fact" that Don Mattingly is a poor to average
>fielder has NOT been proven right yet (although I'm sure it has in some of
>the "stathead"'s minds).  This is an opinion.
>
>> 
>>> I don't think there is a defensive stat that can accurately measure  
>>a
>>> player's performance...
>>
>>Why?  And, for answer you give, why wouldn't this also apply to
>>hitting?  You can argue that you need to watch a player to see
>>how gracefully he fields. But does how gracefully a batter
>>swings enter into how good he is at the plate?  Do you want
>>someone on your team with a .330 BA and .410 OBA who swings ugly
>>or someone with .280 and .330 who swings pretty?
>
>Why?  Because there are too many things that the statistics don't measure
>that are relevent (to me, anyway) such as managers putting shifts on, the
>quality of the team the player is on, the pitching staff of the team the
>player is on, firstbasemen holding runners on, etc.
>

But we don't consider those things when we look at a player's batting
average.  There are some efforts to look at situational stats, but not
to the level of considering whether the opposing manager put a shift on,
whether the first baseman was holding the runner on, whether the manager
told the batter to take a pitch to let a runner steal or swing at a pitch
because the hit and run was on.  We just count the number of opportunities 
the player had (e.g., at bats or plate appearances) and the number of 
successes he had (e.g., hits or times on base) and get a success rate 
(e.g., batting average or on base average).  Defensive average is doing
the same thing with defensive opportunities.  Clearly there are second 
order effects such as the shift and whether the runner was being held on
that affect DA, but there are also second order effects that affect the
offensive stats.  We attempt to correct for general park effects, but not
much more than that.

It's possible that the combined second-order effects are more important
for defense than for offense, but DA certainly represents a major step
in providing a basic, first order look at defensive performance, just
as batting average does for offense.  Just as offensive statistics 
provide us with more information than simply watching a player swing,
DA provides us with more information than simply watching a player
field ground balls or fly balls.  If all of the noise (such as the
things that Eugene Loparco brings up) is really critical, I wouldn't
expect DA to correlate very well from season to season.  That's simply
not the case.  I took the DA numbers that Sherri Nichols posted for 
1988-1991 and correlated DA year-to-year ('88 v. '89, '89 v. '90, 
'90 v. '91) for each league for infielders and outfielders separately.
(Thus, I got 6 correlations for IF and OF.)  The average of the 
year-to-year correlation for the infielders is .6.  Only one of the 
six correlations (AL 1989 v. 1990) wasn't significant at the 0.05% 
confidence level in a t-test.  It only made it at the 1% level 
(correlation 0.44).  For the outfielders, the average correlation is
0.40, significant at the 10% level.  For comparison, correlation for
batting average is somewhere in the 0.4-0.45 range (based on 1984-1987). 
I think that the lower correlation for the outfielders than for 
infielders probably has to do with the greater overlap of areas of 
responsibility in the outfield.

I'm a little troubled by what seems to be a prevailing belief that 
defensive statistics should be required to meet some higher level of
proof than what we implicitly apply to offensive statistics.  It may
have to do with the "age" of the statistics.  Everyone is comfortable
with BA and has a good feeling for what it means.  Just as with the
"new" offensive statistics, the recent efforts at getting at 
defensive statistics, particularly DA, represent an effort to bring
in new information.  Sure, they aren't perfect, but they measure a
large part of the signal and a large part of the difference in 
players' performances.  You can have more confidence in your 
prediction of a infielders' DA for next year, based on previous 
performance, than you can in your prediction of his BA.

Harold
-- 
Harold Brooks                    hbrooks@uiatma.atmos.uiuc.edu
National Severe Storms Laboratory (Norman, OK)
"Here stands baseball's perfect warrior....Here stands baseball's
perfect knight."  Ford Frick about Stan Musial's retirement
Newsgroups: rec.sport.baseball