From: mjw@Xenon.Stanford.EDU (Michael Wolverton)
Subject: Re: Standards for defensive statistics (Was Re: Mattingly...)
Organization: Computer Science Department, Stanford University.
Distribution: na
Date: Fri, 6 Nov 1992 20:29:36 GMT

hbrooks@uiatma.atmos.uiuc.edu (Harold_Brooks) writes:
[Description of batting average as successes / opportunities]
>Defensive average is doing the same thing with defensive opportunities.  

Depends on how you define "opportunities".  One definition of
"opportunities" could be "balls in play", in which case Range Factor
is the defensive analogue of batting average.  What the creators of DA
recognized is that RF's "opportunities" don't even out from player to
player in the league, so they needed to do something to eliminate as
many second-order factors (mostly pitching staff factors) as they
could.  Their new version of "opportunities" was "balls hit in a
player's zone".  What Eugene Loparco brings up is that, even within a
zone around the player, opportunities might not even out (or even come
close to even-ing out) from player to player in the league because of
pitching staff and coaching factors.  My point here is this: there was
a perceived major fault in RF, even though it is merely successes /
"opportunities"; the fault in DA *may* still be a major one, even
though it is also an analogue to batting average.

You bring up a good point that batting average also has these
second-order factors from the player's team which could be playing a
role: coaching instructions (to make intentional outs, e.g.), the
effects that runners on base have on the pitcher and defense, etc.
It's been generally accepted over the years that these factors are
small enough and consistent enough from player to player that they
don't influence a player's average very much.  You could be right that
the age of the statistic has something to do with this general
acceptance.  On the other hand, it could be that a better offensive
analogue to DA would be something like RBIs/ABs -- this is still
"successes" / "opportunities", but everyone would acknowledge that the
second-order teammate factors are playing a *huge* role in determining
this number.

>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.

-Michael

-- 
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Michael Wolverton                       "Is that clear?"
mjw@cs.stanford.edu                     "No, but it's consistent!"
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