From: sloan@cis.uab.edu (Kenneth Sloan)
Newsgroups: rec.games.chess
Subject: Re: USCF Rating
Date: 16 Dec 92 01:06:23 GMT
Distribution: na
Organization: CIS, University of Alabama at Birmingham

In article <3580003@hpcc01.corp.hp.com> selee@hpcc01.corp.hp.com (Lee Say Eng) writes:
>...
>
>If EVERYONE is UNRATED, does that mean that everyone starts from a rating
>of zero (0)?  And as these people play, wouldn't some of them get a 
>negative rating ?
>
>Am I missing something?
>

Yes.  You cited a description of how the USCF systems operates now, in
the steady state.  So..."If EVERYONE is UNRATED" is irrelevant, because
it's not the case.  Every player entering the USCF system earns a rating
by playing against already-rated players.

>
>> 
>> Average these numbers.  (If unrateds play other unrateds, this requires
>> several iterations of the above.)
>> 
>
>Do you mean that if every one of the player is unrated than the above 
>should be iterated a few times 20 games (say 100 games)?
>

No.  There are two senses for the "iterations" suggestion.  

1) in a given event, the unrated players are rated first.  Usually, this
is possible.  Then, you use the POST-event ratings for the unrateds as
their PRE-event ratings when rating the other players.

2) sometimes (rarely) this doesn't work, because there are lots of games
among the unrateds.  So, you first assume that they have a given rating
(say...0).  Rate the tournament.  Note that the unrated's ratings are no
longer 0.  Take the post-event ratings of the unrateds, and make that
their pre-event rating.  Rate the tournament again.  Check to see if the
unrateds; ratings have changed.  If not, you are done.  If so, do it
again...and again...and again, until the unrateds's ratings converge.
Then stop.  You now have ratings for the unrateds which are consistent
with their play in this event (see detailed example, below).

>...
>
>Should a new rating be computed for the two players the moment ONE game
>is completed?
>
>Or can they play a series of games before computing
>their new ratings in bulk.
>

It doesn't really matter.  Ratings are approximations, and hunt for your
current strength.   Calculating rating changes after every game is
usually overkill, but it's not wrong.

>
>
>I have been given the honor/burden of setting a chess team in the
>place that I work.  To encourage more people to take part, we will
>offer prizes to deserving players.  To decide who is deserving, we
>have to come up with a way of rating the performance of the players
>and I thought we might as well use the USCF system to make things
>believable.  Yes, nobody here is rated under any chess organisation,
>and none of us have the chance to play opponents with ratings - we
>have to create some ratings out of nothing.
>

If you really have no reference to the USCF scale (not even one USCF
rated player?), then you have to guess.  In the absence of any other
information other than that this is a group of adults, none of whom are
USCF rated, I would peg the mean rating at 1200.  To get started, give
everyone an "established" rating of 1200.  When new players join the
pool, you can either do the "provisional rating" lead in, or simply give
each new player a 1200 rating.

For a small group, it would probably be instructive to do all of the
computations by hand for awhile, to get a feel for things.  If the group
is too big for that, I have some programs which may help you run you
private rating system.  If you are interested, contact me via email.

>
>Is the FIDE rating system simpler?  It'll be good if I can see what
>the difference is between the two.

To first order, they are identical.

Now - here's the detailed example I promised:
                        Mountain Brook Octagonal
                            23 October 1992
                                  Open
Player                Prev Perf Post R01  R02  R03  R04  R05  R06  R07  Score
  1       David Sloan 0989 0982 1008 W002 W006 F    W003 W005 W007 W004  6.0
  2      Robert Dykes 0000 0754 0754 L001 W003 W005 W007 W004 F    L006  4.0
  3     Juan Martinez 0000 0697 0697 D004 L002 W006 L001 F    W005 W007  3.5
  4       Oscar Price 0000 0583 0583 D003 W005 W007 F    L002 L006 L001  2.5
  5        Paul Dumas 0000 0525 0525 W007 L004 L002 W006 L001 L003 F     2.0
  6     Allen Bunting 0000 0525 0525 F    L001 L003 L005 L007 W004 W002  2.0
  7     Brooks Boylan 0000 0411 0411 L005 F    L004 L002 W006 L001 L003  1.0

Only 1 player has a pre-event rating.  The other ratings were computed
by iterating until their ratings were consistent with the results.  If
*no* player had a rating, then some of the ratings would go negative.
That's OK - but if you don't like negative ratings you could then add a
constant (say 1200) to all of the Post-event ratings.

I wouldn't want to do this by hand - but it's duck soup for a computer.

As a reality check - it turns out that Juan Martinez actually had a
provisional USCF rating of 988/6.  His calculated rating of 697 is not
too out of line - especially considerinng that the one anchor (David
Sloan's 989) is known to be low compared to his true strength.  My
personal assessment is that Juan is no playing at 900 strength, and
David at 1250.  

So...the calculations work reasonably well, considering the amount of
actual truth that goes into them.  It is wise not to put too much faith
in any rating based on fewer than 20-30 games.  Anything less is
semi-organized noise.


-- 
Kenneth Sloan                   Computer and Information Sciences
sloan@cis.uab.edu               University of Alabama at Birmingham
(205) 934-2213                  115A Campbell Hall, UAB Station 
(205) 934-5473 FAX              Birmingham, AL 35294-1170