I played 13/8, figuring that being behind in the race, I should leave the back checker on the 24pt for more contact. Though the 20pt anchor is better than the 21pt anchor, I thought that White's stripped 8pt meant that there was not much danger of her filling in her 5pt any time soon. If White had had more builders for her 5pt, I would have been more inclined to upgrade the anchor.
The rollout favors upgrading the anchor. Nack and Neil seem to be arguing that it's thematic to move the back checker forwards. I have some trouble believing this, given the large race deficit. Below are a couple of variants that I evaluated with XGR+ in an attempt to explore this theory; unfortunately, they're not conclusive.
Nobody mentioned the score. Rule of thumb is that checker play at 3a3a is the same as for money, except possibly when there's an imminent cube turn. However, another exception sometimes occurs when the checker-play decision hinges on a win/gammon tradeoff, since the gammon value at 3a3a is elevated. Here, the pseudocubeless numbers suggest that anchoring results in noticeably fewer net gammon losses while 13/8 doesn't win a lot more games. My rollouts aren't long enough to settle the matter completely but they seem to indicate that the two plays are closer for money than they are at 3a3a.
I'm still inclined to the theory that 24/20 21/20 wins out not because it advances the back checker, but in spite of advancing the back checker, and its main virtue is that it upgrades the 21pt anchor to the superior 20pt anchor, with greater security against getting primed/gammoned.
Mike Corbett was of the opinion that Snowie would run the back checker in this sort of situation more eagerly than it should. I'd be interested in what he thinks of this position.
Rollout of original position:
          
 
        
 | | White is Player 1
score: 4
pip: 140 | | 7 point match | pip: 166
score: 4
Blue is Player 2 |
|
| XGID=----BbD-B---bD---bde-B--A-:0:0:1:41:4:4:0:7:10 |
| Blue to play 41 |
| 1. | Rollout1 | 24/20 21/20 | eq: -0.324 |
| Player:
Opponent: | 41.22% (G:7.07% B:0.34%)
58.78% (G:10.96% B:0.52%) | Conf: ± 0.010 (-0.334...-0.314)
Duration: 6 hours 44 minutes |
|
| 2. | Rollout1 | 13/8 | eq: -0.354 (-0.030) |
| Player:
Opponent: | 41.45% (G:7.45% B:0.48%)
58.55% (G:11.80% B:0.70%) | Conf: ± 0.012 (-0.366...-0.342)
Duration: 6 hours 44 minutes |
|
| |
1 3888 Games rolled with Variance Reduction.
Dice Seed: 2
Moves and cube decisions: 3 ply
|
eXtreme Gammon Version: 1.21, MET: Rockwell-Kazaross
Rollout for money:
| | White is Player 1
score: 0
pip: 140 | | Money session | pip: 166
score: 0
Blue is Player 2 |
|
| XGID=----BbD-B---bD---bde-B--A-:0:0:1:41:0:0:0:0:10 |
| Blue to play 41 |
| 1. | Rollout1 | 24/20 21/20 | eq: -0.305 |
| Player:
Opponent: | 41.41% (G:6.98% B:0.27%)
58.59% (G:10.98% B:0.30%) | Conf: ± 0.011 (-0.316...-0.294)
Duration: 4 hours 34 minutes |
|
| 2. | Rollout1 | 13/8 | eq: -0.314 (-0.010) |
| Player:
Opponent: | 41.51% (G:7.33% B:0.25%)
58.49% (G:11.65% B:0.37%) | Conf: ± 0.013 (-0.327...-0.301)
Duration: 4 hours 35 minutes |
|
| |
1 2592 Games rolled with Variance Reduction.
Dice Seed: 2
Moves and cube decisions: 3 ply
|
eXtreme Gammon Version: 1.21
Variant 1:
| | White is Player 1
score: 4
pip: 140 | | 7 point match | pip: 166
score: 4
Blue is Player 2 |
|
| XGID=----BbD-B---bD---bde-B--A-:0:0:1:32:4:4:0:7:10 |
| Blue to play 32 |
| 1. | XG Roller+ | 13/8 | eq: -0.368 |
| Player:
Opponent: | 41.14% (G:6.90% B:0.20%)
58.86% (G:10.84% B:0.30%) | |
|
| 2. | XG Roller+ | 24/22 6/3 | eq: -0.369 (-0.001) |
| Player:
Opponent: | 41.14% (G:7.07% B:0.23%)
58.86% (G:11.47% B:0.33%) | |
|
| 3. | XG Roller+ | 24/21 6/4 | eq: -0.378 (-0.010) |
| Player:
Opponent: | 40.12% (G:6.55% B:0.20%)
59.88% (G:9.72% B:0.24%) | |
|
eXtreme Gammon Version: 1.21, MET: Rockwell-Kazaross
Variant 2:
| | White is Player 1
score: 4
pip: 140 | | 7 point match | pip: 166
score: 4
Blue is Player 2 |
|
| XGID=----BbD-B---bD---bde-B--A-:0:0:1:11:4:4:0:7:10 |
| Blue to play 11 |
| 1. | XG Roller+ | 21/20(2) 8/7(2) | eq: -0.294 |
| Player:
Opponent: | 42.82% (G:7.22% B:0.20%)
57.18% (G:10.05% B:0.28%) | |
|
| 2. | XG Roller+ | 24/22 21/20(2) | eq: -0.316 (-0.022) |
| Player:
Opponent: | 41.82% (G:6.86% B:0.20%)
58.18% (G:9.82% B:0.24%) | |
|
| 3. | XG Roller+ | 21/20(2) 4/3(2) | eq: -0.335 (-0.041) |
| Player:
Opponent: | 41.88% (G:7.34% B:0.22%)
58.12% (G:10.79% B:0.29%) | |
|
eXtreme Gammon Version: 1.21, MET: Rockwell-Kazaross