From: hbrooks@uiatma.atmos.uiuc.edu (Harold_Brooks)
Subject: In defense of DA (Was Re: Lists: The Best Players In Baseeball)
Organization: NSSL
Distribution: na
Date: Fri, 8 Jan 1993 14:57:10 GMT
Keywords: stars best baseball

In article <1993Jan8.032738.23621@athena.mit.edu> cmk@athena.mit.edu (Charles M Kozierok) writes:
[deletions of a discussion with Valentine]
>
>based on your comments, and the post by Clay where he explains
>what the DA and RF are (thanks, btw), i would have to conclude that
>these statistics definitely are bogus. i don't know why anyone would
>use them to judge much of anything. i know i wouldn't.
>

You're being extremely harsh on DA.  Let's step back a minute and look
at RF and DA.  They both measure the number of balls a player turns 
into an out, but they use different denominators.  RF uses games played
(it can be adjusted by an estimate of innings played), while DA uses 
the number of balls hit in an area near a position (the same area for
each player in the league).  It's equivalent to looking at hitters by
hits per game versus hits per at bat.  RF was developed to be able to
look at historical data and to be used when PO, A, E, and GP were all
that were available, which was the case until about the mid-80s.  DA
requires a lot more data, but gives a much better picture.

One of things I like to see in a statistic is that it is consistent
from year to year for players, that is, that it correlates between
seasons.  Infielder DA correlates better from year to year than almost
any other statistic we have in baseball, such as batting average.  Yet 
we all (or most of us) have an implicit belief that our knowledge of 
what a player has hit in the past allows us to put some bounds on 
what a player will hit in the future.  As I posted some time back,
we tend to have less strong beliefs about defensive statistics, even
though past DA actually tells us more about how someone will field
than past BA tells us about how they will hit.  For an admittedly 
small sample, DA appears to be consistent for players who are traded
or change surfaces of their home fields.

On the Alomar discussion, one of the big things that RF can't pick 
up on is a difference in the number of opportunities.  In '91, only
one AL team had fewer ground balls hit to its second basemen than
Toronto.  For the season, there were ~50 fewer ground balls hit to
Toronto second basemen than to the average AL second baseman.  Since
Alomar turns ~70% of ground balls into outs, that's 35 fewer assists.
  
A second item is that RF rewards the middle of a double play with 
2 plays (one assist, one putout).  Again, in '91, Toronto ranked
13th in the AL in number of ground balls hit to the shortstop and
14th in the league in ground balls hit to 3B in double play
situations.  In total, there were 64 fewer ground balls hit to the
left side of the infield of Toronto in DP situations than to an
average AL infield.  If Toronto was average in turning DPs (36%
on balls hit to short and 23% on balls hit to third), that's another
~35 PO+A that Alomar lost.

A final difference is that DA for infielders only looks at ground
balls, whereas RF counts all putouts, including those in the air.
If, for example, you had a center fielder with a lot of range, he
might take putouts away from a second baseman that an average CF
wouldn't get on balls hit behind second.  Similarly for the right
fielder, 1B, or SS.  I can't really estimate that, but since 
Toronto's RF position led the AL in DA in '91 and Toronto's CF
position was second, it seems likely that Alomar lost some plays 
to them as well.  All in all, Alomar might have lost more than 1/2
of a play per game because the opportunities weren't there for him.

All of the weaknesses in using range factor to evaluate infielders
show up in the case of Alomar.  At that, I don't think range factor
and DA disagree all that often.  It's just that when they do, we 
notice it and the cases tend to stick with us.  Having said that, 
range factor is clearly inferior to DA and is really only used
because of the historical problem and because it's difficult to 
get timely DA numbers.

Harold

-- 
Harold Brooks                    hbrooks@uiatma.atmos.uiuc.edu
National Severe Storms Laboratory (Norman, OK)
"Better to trade a player a year too early than a year too late."
--Branch Rickey