Re: [Bug-gnubg] I'm a bit confused

[Joseph Heled]

This is a position that results in a deep backgame quite often.

I looked at some (10) continuation games after each of the 2 moves. The 
2ply analysis indicates 0ply plays those reasonably well. The cube 
action is probably not as good. It might be that move 1 elicits more 
premature doubles from O, and that is why move 1 is favored. However, it 
is very hard to tell. Many games end up with 4 or 8 cube. I would use 
2ply cube decision for the first cube (do we have such an option at 
all?) to get a little more confidence.

-Joseph

Joern Thyssen wrote:
> On Tue, Jul 29, 2003 at 04:19:01PM -0300, Albert Silver wrote
>
> > I'm trying to understand why move 3 is a -0.147 blunder when its wins
> > are only 0.1% less, it scores more gammons, and loses less gammons and
> > backgammons. 
>
>
> You can't rely on the cubeless numbers when doing a cubeful rollout.
>
> If the game reaches a "double, pass" then the rollout is truncated with
> a cubeful equity of +1 (or -1) and gnubg uses the cubeless numbers from
> the cube decision (which is 0-ply in your case). 
>
> Worst case scenario: you're on roll in a position where you'll be
> doubled out no matter what you roll. If you do a 0-ply cubeful rollout
> then the cubeful equity is -1 and the cubeless gwc is identical to a 
> 1-ply evaluation even though you've selected 1,679,616 trials.
>
> For this very reason I've been on the verge of removing the output
> wins/gammons/backgammons from all evaluations and rollouts! Your
> question is not the first, and certainly not the last! 
>
> Try doing a full cubeless rollout. I imagine that you'll see totally
> different numbers. Your numbers may also be an indication of 0-ply not
> being good enough for the cube decision, so you can try a rollout with
> 0-ply chequerplay and 2-ply cube play.
>
> You may see similar problems for evaluations as well, but as evaluations
> are never truncated then the problem is usually not so large.
>
> Jørn
>
>
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