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Doubling with just 22.3% chance of winning

Posted By: bb766
Date: Thursday, 18 July 2013, at 9:06 p.m.

The score is: You 2, GnuBG 0 (match to 7 points)

Move number 4: GnuBG on roll, cube decision?

You5

1X2X ' ' ' ' ' ' ' ' ' '

 '4O1O ' ' ' ' ' ' ' ' '
GnuBG11
Position ID: DQAA4AUAAAAAAA Match ID: UgngACAAAAAE

GnuBG doubles

This double really surprised me! But, I guess it's pretty simple? If GnuBG loses ATS, it's down 7away 1away. According to its match equity table it would only have 9.28% MWC, far less chance than putting the entire match on the line right now.

Cube decision
4-ply cubeless equity -0.555(Money: -0.555)
0.223 0.000 0.000 - 0.777 0.000 0.000
Cubeful equities:
1.Double, take -0.532
2.Double, pass +1.000 +1.532
3.No double -0.555 -0.023
Proper cube action:Redouble, take

However...If I change the current score to 1 to 0, then GnuBG is down 7away 2away if it loses. Its corresponding MWC is 15.79%, still far below the 22.3% chance it has to win this game. But, it says that's a big error (-0.211) to double.

The score is: You 1, GnuBG 0 (match to 7 points)

Alert: wrong double ( -0.211)!

Cube decision
4-ply cubeless equity -0.555(Money: -0.555)
0.223 0.000 0.000 - 0.777 0.000 0.000
Cubeful equities:
1.No double -0.555
2.Double, pass +1.000 +1.555
3.Double, take -0.766 -0.211
Proper cube action:No redouble, take (12.0%)

Further more, at the original score, several rolls previous to this position it had reached a 27.7% chance. But, 4 ply analysis said it'd be a -0.088 error to double.

So, what is it that this intermediate does not not grasp?

Thanks

ps- I'm using: Version GNU Backgammon 1.00.000-mingw 20130519.

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