(Mike Ogush) wrote in message ...

I wanted to say a bit more about the problems of using a game list to
try to generate a perfect opening book:

1. As Steve Lopez (in the Chessbase Technical Notes) has pointed out
statstics (regarding the frequency a particular move has been played
and that moves performance) can be deceptive.

Say in some opening line White has played 14.Nxf6 in 20 games with a
60% performance rating and as a result Black may have even abondoned
the line that led to the position where white plays 14.Nxf6, then some
enterprising GM deeply analyzes Blacks subsequent play and finds a way
for Black to get an advantage by force. Even if he wins the game
White's performance will only drop to 57% - falsely indicating that
14.Nxf6 is still a pretty good move.

Yes I can see that.

2. One way of getting around the statistical problem is to use a
feature of Bookup called backsolving. Essentially backsolving starts
from the end of all games in a collection and propagates the
evaluation back up the move tree (using +- as the evaluation when
white wins, and -+ as the evaluation when black wins). So if you
backsolved the 21 games with 14.Nxf6 the game where the GM forced an
advantage to Black and won the game would override other games and
give 14.Nxf6 an evaluationj of -+.

Unfortunately backsolving has certain limitations when it relies
solely on game results: A player can lose on time in a won position.
A player could play a three-fold repition allowing a draw in a won
position. A player can reach a won position and through a series of
mistakes throw away the win and even the draw and eventually lose.
This can be accounted for by doing extensive analysis of games rather
than relying on their results to get a good evaluation for a given
position.

3. Lastly even if the evaluations for a moves are completely accurate,
they can result in situation where the computer as white has an
advantage (+/-) or even a won game (+-) when it leaves its opening
book, but the program does not know how to play the subsequent
positions. This is often true if the advantage is positional and
requires a player to do long term strategic planning in order to
realize the win; this is eactly what most programs are notoriously
bad at. I have seen humourous situations where computers drop out
book with a position that a human being recognizes as completely won
and the computer's evaluation of the position is that its opponent has
a winning advantage.

Because of the above, many commercial chess program vendors have an IM
or even a GM on staff to help tweak the opening book. A few years ago
the chess site for Rebel would include some notes about the tweaking
done to the opening book for Rebel; I don't know if they still do
this. In some of the man-machine matches of the last few years with
Kaspraov and other GMs the reports on the matches would talk about
how the opening book needed to be modified to stay away from close
positions where a GMs ability at strategic maneuvering would give him
the advantage.

When a computer is playing humans below master level the errors in the
opening book aren't so critical. Although, we do see people bragging
how they can beat a strong program time after time, which is only
because they have found a new flaw in the programs opening book.



Mike Ogush

Thanks for the detailed explanation Mike. It is clear there is a lot
more to the opening book than I thought. From my own point of view,
crafty can always beat me unless I make serious attempts to weaken it,
so making a strong machine to play aginst does not bother me that
much. I was however keen to use the database programs and chess engine
in combination to find good moves to play in the opening. But its
clear my knowledge it too limited at this point to really comment
further on any skewing that may or may not occur.

David Kirkby