White - Pips 78 (-130), Match -2 -4
Black - Pips 208 (+130), Match -4 -2
Black to Play 6-2
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Black trails 3-5 in a 7-point match.
(a) How do you play 6-2?
(b) Move a checker from the 24 point to the midpoint, now how do you play 6-2?
The main decision here is to hit or not.
XG says not to hit in (a) (hitting is -0.05 error), but to hit in (b) (not hitting is a -0.04 error). I used a 1296-game rollout, standard XG settings, over all legal moves (since there aren't that many).
Why is this?
If you do not hit, then White escapes and you are playing an ace-point game.
If you hit, then you can hope to contain the checker (probably after some more hits) and force White to release the prime. If White eventually escapes anyway, you are back to the ace-point game. But Black could also get hit, locking him out for a few turns that could have been spent building his board for an eventual shot.
The difference between (a) and (b) is that in (b), Black has an extra checker to play with. Can that really make that big a difference?
Here is one reason the rollout could go wrong. If Black hits now, then there will be many decisions in the near future about whether to hit or not. If XG gets any of those wrong, the rollout decision may end up wrong.