Backgammon Snowie

Snowie

Snowie cube evaluation

From:   Kit Woolsey
Address:   kwoolsey@fibs.com
Date:   4 September 2007
Subject:   Snowie cube evaluation
Forum:   GammOnLine

I have often thought that Snowie's cube decisions from rollouts were suspect, and did not properly reflect the results of the rollouts. I couldn't place my finger on exactly what was wrong. This last weekend, Neil demonstrated to me just what the problem is, with the following example: Money game, cube in center. X on roll has 4 checkers on ace point and 1 checker on 5 point. O has 5 checkers on ace point. 13 14 15 16 17 18 19 20 21 22 23 24 +---+---+---+---+---+---+---+---+---+---+---+---+---+ | | | O | | | | O | | | | O | | | | O | | | | O | +---+ | | | | | 1 | | | | | +---+ | | | X | | | | X | | | | X | | | | X X | +---+---+---+---+---+---+---+---+---+---+---+---+---+ 12 11 10 9 8 7 6 5 4 3 2 1 X doubles. Should O take? It is clear that O's recube vig is tiny. The only way O could get a recube in is if X rolls two consecutive 2-1's and O doesn't roll a double. If this happens X can cash and not have to sweat O rolling doubles on the last roll. On any other variation X will take at least 3 checkers off in two rolls, so either O rolls a game-winning doubles or he loses but he doesn't get to recube. According to Snowie, X will win this game played to conclusion 77.6% of the time. I believe this is an accurate figure. Therefore, O has a clear pass, since O wins substantially less than 25% of the time and his recube vig is virtually non-existent. However, Snowie evaluates this as double-take! What is wrong? According to Neil, the programmers do not attempt to determine the likely efficiency of a redouble when evaluating a pass/take decision. Instead they assume an average efficiency for all positions, which results in the break-even point (assuming gammons aren't in the picture) of about 21.5%. Since O wins greater than 21.5%, Snowie thinks it is a take. This flaw undoubtedly leads to very inaccurate cube evaluations by Snowie. If the position is such that an efficient recube is very unlikely to occur, Snowie will be taking too often. On the other hand, if the position is such that an efficient recube is more likely than normal, Snowie will be passing too often. I have never respected Snowie's cube evaluations even after a rollout. Instead, I look at that wins, losses, gammons, and backgammons. These I can trust, assuming that Snowie can play the position decently which is almost always the case. I then use these numbers to form my own cube assessment. Kit

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