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

From: Joern Thyssen
Subject: Re: [Bug-gnubg] I'm a bit confused
Date: Tue, 29 Jul 2003 21:13:40 +0000
User-agent: Mutt/1.4.1i

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.


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