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Re: [Bug-gnubg] Re: Strange FIBS ratings

From: Joern Thyssen
Subject: Re: [Bug-gnubg] Re: Strange FIBS ratings
Date: Mon, 8 Sep 2003 14:52:06 +0000
User-agent: Mutt/1.4.1i

On Mon, Sep 08, 2003 at 03:03:55PM +0200, Jim Segrave wrote
> On Mon 08 Sep 2003 (11:04 +0000), Joern Thyssen wrote:
> > On Mon, Sep 08, 2003 at 11:36:33AM +0200, Jim Segrave wrote
> > 
> > Kees' experiments show that cube decisions errors don't weigh as much as
> > chequer play errors. I can't offer any explanation for this, other than
> > gnubg's chequerplay is much better than the cube play???
> >From a comment in the thread on GammOnLine:
> ================
> seems to me that checker play errors in real matches represent are
> always an irretrievable loss of equity, while cube errors may or may
> not matter, depending on the flow of the game (5 missed marginal
> doubles with an eventual correct double/take), and opponent's error
> (too good to double, but he took.) Objective cube errors may not even
> be errors (there's little play-the-opponent in checker play, but a lot
> in cube action). Further, it seems that cube errors against weaker
> opponents are relatively less costly than cube errors against stronger
> opponents (against a weakie I can recover from a bad take and gammon
> loss in the first game of a 5-point match, or choose to play the whole
> match semi-cubeless, or take "passes" that opponent's checker errors
> make takes -- in gnu's eyes I'll be a "casual cubist" in all cases).
> ================

I tried the same argument myself before writing my comment, but wasn't

If gnubg says that you lose 0.1 by missing a double, why do you lose
less MWC than had it been a 0.1 chequer play error? The argument above
seems to be that you often can correct your error on the next roll, but
I don't follow that: you'll only be able to correct a fraction of a all
games, otherwise gnubg would not say it was a double! 

Can someone help me understand?

The argument about playing against weaker players is of course valid.
gnubg calculates cube decisions assuming "perfect" play.

My theory would be that as we approach perfect play chequer play errors
and cube errors should be weighted equally. With non-perfect play cube
errors will be weighed less, hence my comment about gnubg being louse at
cube play!

> Which also leads to the observation that you can only make a given
> chequer play error once in a game, you can accumulate say .050 or so
> per missed double for several moves (try failing to double when your
> opponent is post Crawford to see how big a cube error you can
> generate).
> If the rating calculation is done by the average cube error, then the
> fact that the average is calculated by only considering actual or
> close decisions, which can keep the cube average error higher.

gnubg uses two different formulae for the rating loss from cube errors
and chequer play errors. The abs. rating given is the sum of these plus
some arbitrary offset.


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