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Confused by move

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05-14-2019 , 07:02 PM

seanwhat

stranger

Join Date: Feb 2015 Posts: 3

I do not understand why xg prefers 13/4 over 13/10 13/7 in this position. All of my backgammon knowledge tells me I should be playing the latter to leave fewer hits for my opponent. A large rollout puts it at -0.064 difference (similar to what was picked up by xg anyway), so this isn't just a small discrepancy. Can anyone shed some light on this?

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05-15-2019 , 01:22 AM

_Z_

adept

Join Date: Jul 2004 Posts: 720

By staying back, white gets hit 4/36 more, but black also will get hit sometimes, as black is forced to leave a shot with 61, 62, 63, 51, 52, 53. This works out to about a 4/36 chance of getting hit too. Hits are worth more for White owing to his stronger board. Also by staying back, white slows down black on 32 and 21.

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05-16-2019 , 04:09 PM

_Z_

adept

Join Date: Jul 2004 Posts: 720

My analysis is wrong it seems or at best incomplete. Cubeless win percentage is about the same between the plays. So I think the hits are not worth more for White. White's stronger board is a factor, but White is essentially up in the race (due to Black's wastage) so that factor goes the other way. Hits are better for the player down in the race.

So I now think the main difference is staying back allows White to get in a decent cube after Black's bad numbers (shot leaving numbers and the 21 and 32 slowdown numbers). I guess in a sense the hits are worth more for White, but only because he gets to double before them.

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05-16-2019 , 06:05 PM

seanwhat

stranger

Join Date: Feb 2015 Posts: 3

Quote:

Originally Posted by _Z_

So I now think the main difference is staying back allows White to get in a decent cube after Black's bad numbers (shot leaving numbers and the 21 and 32 slowdown numbers). I guess in a sense the hits are worth more for White, but only because he gets to double before them.

This would make sense i guess. It's really tough this one. I feel like this move is something no human would ever play.

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05-16-2019 , 06:53 PM

_Z_

adept

Join Date: Jul 2004 Posts: 720

It's not crazy to find moves like this as it comes up occasionally. Often when it happens, the player volunteering the extra shots is both down in the race and has the stronger board, so both factors point in the same direction. Here it's cubeless better (by a lot) to pay the extra shots despite the fact that White gets hit less than the extra 4/36 that Black is paying.

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05-17-2019 , 05:00 AM

bleep69

grinder

Join Date: Sep 2008 Posts: 483

The problem in finding these moves over the board is that generally if you get them wrong (which happens to me often when I try what seems to be an "innovative" play), they tend to be blunders, whereas the stable, more obvious move is a small error. This discourages me from trying these "innovative" plays.

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05-18-2019 , 07:47 PM

Eric

Carpal \'Tunnel

Join Date: Oct 2009 Posts: 13,853

I am, by no means, more than basically competent at backgammon. I wonder if part of the difference between the two choices is that with 13/4, you are left with only one checker to bring home, when you are hit, rather than having to use a roll to bring that last checker in from the bar point.

If black doesn't hit, does white have a double now for some of the rolls?

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06-16-2019 , 01:46 AM

satellite

newbie

Join Date: Jun 2019 Posts: 23

[QUOTE=Eric;55129609 with 13/4, you are left with only one checker to bring home, when you are hit, rather than having to use a roll to bring that last checker in from the bar [/QUOTE]

I suppose that might be a factor, although when you are hit your cubeful race equity is pretty small. Also, after contact one would always bring both down from the 13, no?

Maybe the strong board adds some gammon equity when you hit back with a joker?

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06-17-2019 , 08:51 AM

Robertie

old hand

Join Date: May 2004 Posts: 1,954

After 13/10 13/7 Black has no really bad rolls. He hits with aces and goes by with all larger numbers.

After 13/4 Black has more hits (15 instead of 11) but also 12 bad numbers that leave a shot (61, 62, 63, 51, 52, and 53). White hits about 1/3 of those. The existence of the bad numbers for Black is what swings the play.

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