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Old August 19th 07, 10:24 PM posted to rec.games.chess.computer,rec.games.chess.misc
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Default GM Joel Benjamin vs Rybka today.Give us your break down Helpbot




help bot wrote:

David Richerby wrote:

Fred wrote:

Could you explain the "contempt factor", please?


Suppose a chess engine evaluates drawn positions as zero, which seems
natural enough. This means that, in a position where the evaluation
is, say, -0.02 (against the computer), the computer would accept a
draw by repetition if it saw nothing else better than -0.02. In
practice, this isn't a good idea as it means that the engine will play
for a draw as soon as some random little feature of its evaluation
function causes the evaluation to go ever so slightly negative.

So, what the engines do is score a draw as something a little below
zero; say -0.20. This means that the engine will only play for a draw
if it is at a noticeable disadvantage. Of course, the difference
between -0.22 (play for a draw if there is one) and -0.19 (play for
the win!) is also essentially noise in the evaluation function. But
at least the engine can be sure it has some kind of disadvantage at
both scores and that this disadvantage isn't overwhelming.


In normal play (no odds), even a score of -0.2 would be a poor
choice, for many grandmasters would have little trouble obtaining
this small an advantage, especially as White. For that matter,
many lesser players (incl. chess programs) could easily luck into
such an edge during the opening before the real show begins, so
Rybka would effectively cheat itself by steering for draws here.

As the world's strongest program, it would seem better to use a
contempt factor of, say, 0.8, except when facing super-GMs or
programs like ZapChess. Heck, against patzers, a contempt
factor of 3 or 4 might even do no harm!


On the other hand, Rybka could reason that it being down 2 points
is a pretty good indicator that it was wrong about the opponenet
being a Patzer...

--
Guy Macon
http://www.guymacon.com/

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Old August 20th 07, 01:19 AM posted to rec.games.chess.computer,rec.games.chess.misc
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Default GM Joel Benjamin vs Rybka today.Give us your break down Helpbot

Guy Macon wrote:

On the other hand, Rybka could reason that it being down 2 points
is a pretty good indicator that it was wrong about the opponenet
being a Patzer...


Or, Rybka could reason that, since the opponent is a patzer it is better
to navigate a path THROUGH objectively lost positions in order to reach
a won position - counting on the opponent to NOT see the chances offered
to him. This might lead to a win, where playing *perfect* moves will
lead only to a draw.

Chess, like many games, is played on many levels. A player who decides
that the game is objectively drawn, and as a result forces play down
sterile, drawn lines because "it's best", will never be highly rated.

Consider the junk heap of abandoned opening lines. Some lines are
abandoned because they are inferior. Others are abandoned because they
are too well analyzed. They may, in some narrow sense, be "optimal"
- but they are not good practical weapons (for the grandmaster).

And then again - they actually ARE good practical weapons for the patzer.

Sam Sloan's opening repertoire seems to be based on a very high
"contempt factor". He enters objectively horrible positions - on the
theory that "there are traps here, and my opponent will surely blunder".
Sometimes, this contempt is justified. Here, for example, his
opponent misses a mate in 5 AND a mate in 4. Still, perhaps the
contempt factor was set just a tad too high...

http://www.cis.uab.edu/sloan/ChessGames/SloanVsSloan/

--
Kenneth Sloan
Computer and Information Sciences +1-205-932-2213
University of Alabama at Birmingham FAX +1-205-934-5473
Birmingham, AL 35294-1170
http://www.cis.uab.edu/sloan/
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