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

From: Joseph Heled
Subject: Re: [Bug-gnubg] I'm a bit confused
Date: Wed, 30 Jul 2003 09:47:12 +1200
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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.


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

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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