GNU Backgammon Position ID: sO3uAACwu3sAAA
Match ID : cAkAAAAAAAAA +24-23-22-21-20-19------18-17-16-15-14-13-+ O: Kit Woolsey | O O | | O O O O | 0 points | O O | | O O O O |
| | | O O O |
| | | | | | | | | |BAR| |v (Cube: 1) | | | |
| | | X |
| X | | X X X | | X X | | X X X | On roll | X X | | X X X | 0 points +-1--2--3--4--5--6-------7--8--9-10-11-12-+ X: GOL Readers
Rollouts indicate it's not quite a double yet, with 71.21% GWC.
2-ply and 4-ply cube see 71.18 and 71.32% GWC and also say not quite a double.
0-ply cube sees LESS winning chances, only 70.74%, yet it doubles (0.0243 difference even).
So 0-ply doubles because it overestimates volatility, whereas it actually underestimates GWC here.
Could it be that some parameters used for 0-ply cube decisions need a bit more fine-tuning? This is a very simple long race position where you'd expect volatility to be fairly predictable just by the pipcounts.
Here's the GNUBG output, note that I use a 1-sided DB to the ninepoint so your output may differ slightly, but it seems irrelevant for this issue.