Small Cube Error Reported by XGR++. Why?
Posted By: Henrik Bukkjaer
Date: Friday, 7 February 2014, at 12:23 p.m.
This is a position from a match I have just reviewed for botnorm approval in the DBgF.
Freddie Noer played 1.98 (XGR++ Gigantic) in a live clocked 17-pointer, in a round of the Danish Team Championships vs. Henrik Hansen (a strong intermediate player, who would be expected to play a PR between 5 and 8).
What I just stumbled on, has nothing to do with the analysis of the actual match. But it's a position during post-Crawford play, where Freddie's opponent doubles at his second opportunity (in what XGR++ evaluates as an "Automatic Double / Take"):
is Freddie Noer
17 point match pip: 156
is Henrik Hansen
XGID=-a--B-D-C---fD---c-eA---A-:0:0:-1:00:16:8:0:17:10 on roll, cube action?
Analyzed in XG Roller++ No double Double/Take Player Winning Chances: 47,51% (G:9,53% B:0,29%) 47,44% (G:9,76% B:0,32%) Opponent Winning Chances: 52,49% (G:14,13% B:0,55%) 52,56% (G:14,51% B:0,57%) Cubeless Equities -0,035 +0,147 Cubeful Equities No double: +0,144 (-0,003) Double/Take: +0,147 Double/Pass: +1,000 (+0,853) Best Cube action: Double / Take
eXtreme Gammon Version: 2.10, MET: Kazaross XG2
Now, my question is this: Why would XGR++ give a penalty of 0,003 here, to a player NOT doubling?
I would certainly not double this position myself (well maybe I would in this particular case playing Freddie Noer, but for 95% of my opponents I wouldn't).
Looking at the two strongest exchanges that could happen from here (33 or 44, followed by a dance), XGR++ correctly evaluates those as Double/Take, thus no market loss in the position.
The odd thing is, that XGR++ provides the worst evaluation of all, from 1-ply to 7-ply all mini rollouts included. I've seen this before in post Crawford game cubes - is it in general a problem related to XGR++ few games when looking at positions that are about potential market loss?
Especially it surprises me that XGR++ does worse than XGR+, since they both operate with variance reduction, and have the same truncation point (7 moves).
I cannot recall how many games XGR++ and XGR+ rolls out, maybe that's where the answer lies? Anyone have those numbers?
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