[Computer-go] Question on 0.5-wins

Richard Lorentz lorentz at csun.edu
Fri May 31 07:08:58 PDT 2013

Hi, Ingo.

Indeed it is a puzzle. As you well know (and you have commented on in 
the past) my Amazons program, Invader, displays the same behavior -- 
including during self-play. Since MCTS, under most implementations, is 
truly oblivious to the margin of victory the only conclusion one can 
come to is that, for some reason (?!), close finishes end up with 
slightly higher win rates. I suspect it has something to do with the 
structure of the tree some levels down from the root. For example, 
perhaps positions that are "winning big" also allow slightly more 
chances for a catastrophic loss as well. But it's hard to make a cogent 
argument to that effect.

You got me thinking about this problem as it relates to Wanderer, my 
MCTS Breakthrough program. It is not clear what "margin of victory" 
would mean in the case of Breakthrough, but one reasonable definition 
might be the difference of the distances to goal for each player. It 
does not appear that Wanderer prefers closer games, but maybe that's 
because all games tend to be very close anyway. The same with Havannah. 
I don't think I see many "intentionally" weak moves that keep the game 
close. Maybe this issue manifests more conspicuously in games involving 


On 05/31/2013 03:27 AM, "Ingo Althöfer" wrote:
> Hello,
> especially in the early years of Monte-Carlo Go it
> was often observed in games between MC(TS)-bots and humans
> that bots won by the smallest possible margin, 0.5 points.
> We all know that this is not a bug but a feature ;-)
> For a long time it was my impression that this phenomenon
> was typcial only for bots-vs-humans, but not for
> MC-bots vs. MC-bots. But now experiments with other games
> make me believe that wins by small margins happen often also
> for MC-bots against each other.
> Who has experiences or explanations for this (in Go)?
> Ingo.
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