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A blunder I can't understand

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01-30-2014 , 10:52 AM

Epitaxi

stranger

Join Date: Jan 2011 Posts: 3

It's a game to 3, 2-away/2-away and doubled. I can not understand why hitting and slotting can be a blunder compared to hitting and leaving a blot in my nest.

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01-30-2014 , 11:23 AM

yogiman

adept

Join Date: Jun 2012 Posts: 784

It's again about timing. White likes to be hit, so that black is more likely to ruin his homeboard.

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01-30-2014 , 11:24 AM

Aaron W.

Carpal \'Tunnel

Join Date: Sep 2002 Posts: 30,132

My first guess is that it has to do with the distribution of your spares for bearing in against a double anchor, and the fact that gammons don't matter. It's not like you're in a hurry to make the 2 point here. (Not that I'd have played 15/9 over the board -- I would have done exactly what you did.)

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01-30-2014 , 11:25 AM

Aaron W.

Carpal \'Tunnel

Join Date: Sep 2002 Posts: 30,132

Quote:

Originally Posted by yogiman

It's again about timing. White likes to be hit, so that black is more likely to ruin his homeboard.

I'm not sure I buy this. Black is already on the bar, and sending another checker to the bar with 8/2* doesn't help the timing at all.

Edit: If the 2 point were open, would you still play 8/2 in order to get hit?

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01-31-2014 , 10:45 AM

yogiman

adept

Join Date: Jun 2012 Posts: 784

Quote:

Originally Posted by Aaron W.

I'm not sure I buy this. Black is already on the bar, and sending another checker to the bar with 8/2* doesn't help the timing at all.

Black has a 75% chance to enter. White is waiting to hit the blot on the 13 point. With three blots on the bar, the chance to be hit is presumably 1- 2/3 x 2/3 x 2/3 = 19/27 is about 70%. While circulating there is also still the possibility to make a full 6-prime.

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Originally Posted by Aaron W.

Edit: If the 2 point were open, would you still play 8/2 in order to get hit?

At these kind of positions I trust the bot unconditionally.

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01-31-2014 , 01:06 PM

Aaron W.

Carpal \'Tunnel

Join Date: Sep 2002 Posts: 30,132

Quote:

Originally Posted by yogiman

Black has a 75% chance to enter. White is waiting to hit the blot on the 13 point. With three blots on the bar, the chance to be hit is presumably 1- 2/3 x 2/3 x 2/3 = 19/27 is about 70%. While circulating there is also still the possibility to make a full 6-prime.

I'm unsure as to why you think the number of blots on the bar matters, or what this calculation represents. I also do not see the benefit of making a full 6-prime in this position.

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At these kind of positions I trust the bot unconditionally.

Trusting the bot's decision is one thing. Coming up with a rationale for it is quite another. I can agree that hitting loose is best. I can't say that the reason that you've given makes sense to me.

I'm at work and don't have access to XG. I would be curious to see what would happen if the blot on the 2 point was moved to either the 1 or 3 points.

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01-31-2014 , 06:07 PM

Taper_Mike

adept

Join Date: Sep 2009 Posts: 1,030

Black - Pips 111
Black to Play 6-4

I moved the blot on the 2pt forward for the unlimited games variation above. Slotting the 2pt still comes out on top. According to my 5k rollout below, the decision is not close. The second-best play trails by 0.06. Making the 2pt is a 0.2 error.

This sort of play is not uncommon in backgame defense. With seven White checkers locked up up behind a five-point prime, Black has little to fear after slotting the 2pt. More dangerous would be to freeze his opponent by making the 2pt, and thereby increase the chance that he would dance. That said, finding—and making—such plays, especially when given a “natural” alternative as we are here, is another story.

Many other variations produce the same top play in XGR++ evals. With a roll of 63, for instance, slotting the 2pt above is better than slotting the 9pt.

Thanks to Epitaxi for sharing such an interesting position.

Mike

Code:

XGID=ac-cBBCCC----a-B---bbc----:1:-1:1:64:0:0:0:0:10

X:Player 1   O:Player 2
Score is X:0 O:0. Unlimited Game
Pip count  X: 111  O: 209 X-O: 0-0
Cube: 2, O own cube
X to play 64

    1. Rollout¹    15/9 6/2                     eq:+0.6039
      Player:   64.99% (G:40.96% B:7.86%)
      Opponent: 35.01% (G:2.37% B:0.12%)
      Confidence: ±0.0078 (+0.5960..+0.6117) - [100.0%]
      Duration: 30 minutes 09 seconds

    2. Rollout¹    15/11 8/2                    eq:+0.5444 (-0.0595)
      Player:   62.85% (G:39.90% B:7.98%)
      Opponent: 37.15% (G:2.27% B:0.10%)
      Confidence: ±0.0075 (+0.5369..+0.5520) - [0.0%]
      Duration: 30 minutes 34 seconds

    3. Rollout¹    15/9 8/4                     eq:+0.5072 (-0.0967)
      Player:   61.98% (G:38.89% B:7.84%)
      Opponent: 38.02% (G:2.57% B:0.11%)
      Confidence: ±0.0075 (+0.4997..+0.5146) - [0.0%]
      Duration: 28 minutes 14 seconds

    4. Rollout¹    15/11 15/9                   eq:+0.5011 (-0.1028)
      Player:   62.08% (G:38.71% B:7.12%)
      Opponent: 37.92% (G:2.58% B:0.12%)
      Confidence: ±0.0069 (+0.4942..+0.5080) - [0.0%]
      Duration: 26 minutes 16 seconds

    5. Rollout¹    15/5                         eq:+0.4455 (-0.1584)
      Player:   59.86% (G:37.99% B:7.77%)
      Opponent: 40.14% (G:2.48% B:0.10%)
      Confidence: ±0.0073 (+0.4381..+0.4528) - [0.0%]
      Duration: 25 minutes 39 seconds

    6. Rollout¹    8/2 6/2                      eq:+0.4074 (-0.1965)
      Player:   57.98% (G:37.98% B:8.75%)
      Opponent: 42.02% (G:2.45% B:0.10%)
      Confidence: ±0.0076 (+0.3997..+0.4150) - [0.0%]
      Duration: 26 minutes 04 seconds

    7. Rollout¹    8/4 8/2                      eq:+0.3825 (-0.2213)
      Player:   58.75% (G:36.18% B:7.64%)
      Opponent: 41.25% (G:3.93% B:0.19%)
      Confidence: ±0.0075 (+0.3750..+0.3901) - [0.0%]
      Duration: 30 minutes 28 seconds

¹  5184 Games rolled with Variance Reduction.
   Dice Seed: 37408801
   Moves: 3-ply, cube decisions: XG Roller

Rollout by Taper_Mike
2014-Jan-31
eXtreme Gammon Version: 2.10.199.2658
Flashcard PositionID 000894.xgp

Last edited by Taper_Mike; 01-31-2014 at 06:21 PM.

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02-02-2014 , 01:33 PM

Epitaxi

stranger

Join Date: Jan 2011 Posts: 3

I want to thank you for taking your time with answering and discussing the situation, I appreciate it!

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08-27-2014 , 03:04 PM

peachpie

centurion

Join Date: Aug 2014 Posts: 182

Sorry for the necro. I am reading through old threads and this one caught my interest.

If I think about it slowly, I can understand that making the 2 point is not good (in either the original position or Mike's similar construction). But that begs the question: If I don't want to make the 2 point, why bother slotting it? If I want to hurt black's timing, why hit at all?

At the table I admit I would have made the two point. But, having learned this lesson, I would apply it by playing 15/5 or 15/9 15/11.

???

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08-27-2014 , 05:02 PM

#10

Robertie

old hand

Join Date: May 2004 Posts: 1,954

You slot the 2-point so that you'll be hit if Black rolls a 2 next. Then your checker has to come all the way around the board while Black's position collapses if he doesn't roll some 6s. The chance that your blot gets hit and then never gets out is minimal.

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