Forum Archive : Snowie

Snowie cube evaluation

From:   Kit Woolsey
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.

Did you find the information in this article useful?          

Do you have any comments you'd like to add?     



Announcement  (Olivier Egger, Apr 1998) 
Checker-play-according-to-score bug  (Peter Schneider+, June 2001) 
Error rates  (Gregg Cattanach, Oct 2000) 
Hints and questions  (Achim Müller+, Aug 1998) 
Luck calculation  (Gregg Cattanach+, Dec 1999) 
Questions and answers  (David Montgomery, Dec 1998) 
Running in low priority  (lmfback+, Oct 2004) 
Snowie 4.0  (SnowieGroup Info, Oct 2002) 
Snowie 4.3 update  (Gregg Cattanach, July 2005)  [GammOnLine forum]
Snowie cube evaluation  (Kit Woolsey, Sept 2007)  [GammOnLine forum]
Snowie vs GNU  (Stanley E. Richards+, Oct 2005)  [GammOnLine forum]
Snowie vs. Jellyfish  (Mark Driver, Apr 2001) 
Snowie vs. Jellyfish  (Daniel Murphy, Oct 2000) 
Snowie vs. Jellyfish  (Gregg Cattanach+, Sept 2000) 
Snowie vs. Jellyfish  (Wayne Crookes, Jan 1999) 
Snowie vs. Jellyfish  (Kenneth M. Arnold+, May 1998)  [Long message]
Terminology  (Alexander Nitschke, Sept 1998) 
Using rollouts  (Michael J. Zehr+, Oct 1998)  [Long message]

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