Re: Why such discrepancy for the ranked (obviously wrong) 1st choice between 4 ply and simulation ?

[Timothy Y. Chow]

Pierre Zakia wrote:

> In the position here below, I am curious to understand what in Gnubg 
> engine yields such discrepancy between 4 ply and roll out.
>
>
> GNU Backgammon  ID de position: G27HAAiY2+ABAw
>                  ID de match   : MAHyAAAAAAAE

Thanks for posting this position---it's very interesting!

First let me mention that eXtreme Gammon (XG) finds this position tricky 
as well.  XG 3-ply plays 13/9(2) 13/5.  (XG and GNU use slightly 
definitions of "ply" I think, but that's not important here.)  So I don't 
think that 13/9(2) 13/5 is "obviously wrong."  Two things to note:

1. 13/9(2) 13/5 wins more gammons than 20/16(2) 13/9(2), so it's probably 
the better play at certain match scores.

2. Suppose you change the position slightly---instead of having the cube 
centered, have X own the cube on 2.  I think that if you roll it out then 
you'll see that 20/16(2) 13/9(2) comes out ahead by a much smaller margin. 
So part of the reason that 20/16(2) 13/9(2) comes out so far ahead in your 
original position is the somewhat subtle fact that O is hoping to double 
next turn---but if O gets hit then she won't be able to double.  So with 
the cube in the middle, O is much more cautious about leaving shots.

Anyway, I don't think that this is a fault of the software.  It's just 
that the position is more subtle than you may have thought at first 
glance.

Tim