From: hbrooks@uiatma.atmos.uiuc.edu (Harold_Brooks)
Subject: Re: Standards for defensive statistics (Was Re: Mattingly...)
Organization: NSSL
Distribution: na
Date: Sat, 7 Nov 1992 20:00:09 GMT

In article <1992Nov6.202936.19679@CSD-NewsHost.Stanford.EDU> mjw@Xenon.Stanford.EDU (Michael Wolverton) writes:
>hbrooks@uiatma.atmos.uiuc.edu (Harold_Brooks) writes:
[deletions]
>
>>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.  
>
>I don't disagree with a lot of the things you said in this article,
>but I don't buy your logic here.  The factors that Eugene brought up
>-- pitching, coaching, park effects -- will tend to stay the same for
>a given fielder from year to year.  Even if DA were measuring
>*nothing* but "noise" -- i.e., it had nothing to do with fielding and
>was measuring only pitching and park effects -- we'd still expect it
>to correlate well from year to year.  What we need to look at is how
>well it correlates in players who switch teams, especially those who
>switch from grass to turf.  I've heard third-hand that this has been
>done a long time ago, but I haven't seen the numbers.  I'm not sure if
>there are enough cross-team players in the data Sherri posted to give
>us meaningful results.

True, but the external things stay the same for batters also.  In
fact, they're more constant for batters than for fielders.  If, for
example, the Cardinals traded Bob Tewksbury to the Red Sox for Roger
Clemens, the kind of pitching the Cardinal defense played behind would
be dramatically different next year for ~15-20% of the innings.  The
changes that an individual NL batter would see would be small, at most
4% of his at bats would be against Clemens next year.  I have a hard 
time believing that the external things (pitching staff, coaching, 
etc.) aren't more consistent from year-to-year for batters than 
fielders.  That's why I find the higher correlation for DA to be so
intriguing.  People seem to accept that batting is a consistent skill
and that a fundamental measure of that skill, BA, should be relatively
predictable through the years.  On the other hand, they also believe
that defense is consistent, but when a measure of it appears that
is more predictable than BA, they wring their hands and say, "Oh,
it can't be measuring everything.  There are all these problems 
with it."  

As for the effect of changing teams, I issue the following anecdotal 
evidence warning--
DAs for some players changing teams (and surfaces) in 1991:
Player      1988   1989	  1990  1991
Alomar      .706   .705   .706  .719
Fernandez   .728   .699   .643  .655
Pendleton   .692   .685   .631  .689

Fernandez looks like a player losing his defensive skills, while 
Alomar and Pendleton are consistent through the years, except for 
Pendleton's awful 1990 when the Cardinals had given up on him and
vice versa.  FWIW (which may not be much).

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