[Computer-go] Question on 0.5-wins

Don Dailey dailey.don at gmail.com
Sat Jun 22 12:20:28 PDT 2013

Without looking this over too critically,  it seem like a very reasonable
explanation to me.    It does seem to happen more often than I would expect
it to from random choices and one explanation if this is true is that
something is happening in the tree to encourage it - something like what
you are saying perhaps.


On Sat, Jun 22, 2013 at 3:03 PM, Richard Lorentz <lorentz at csun.edu> wrote:

> I have a thought as to why MCTS bots might prefer to win by smaller
> margins.
> Imagine a situation where a bot has a choice between moves a and b, where
> move a would allow a large win and move b keeps the game much closer.
> Consider the MCTS tree for move a. It is usually the case that a move that
> wins by a large margin also means that many/most of the opponent's moves
> are bad -- and equally bad. In this case the MCTS tree will grow so that
> each of these many possible, equally bad opponent responses gets about the
> same number of visits. Assuming the bot continues to prefer large wins down
> the tree, at each opponent move there will be many equally bad choices so
> the MCTS tree produced will be fairly "bushy" because it will be visiting
> all of these equally bad moves somewhat uniformly.
> On the other hand, in the case of move b, the opponent will now have a
> more even distribution of good and bad moves. MCTS will focus its attention
> on the good moves, of which there are fewer, so now the tree will be less
> bushy and so will grow (considerably?) deeper.
> With the move b tree being deeper, this will mean that most of the random
> playouts will start later in the game and will, presumably, mean they will
> be more reliable. Thus, more wins will propagate up the tree giving move b
> a higher win count. Finally, assuming we are selecting the move with the
> highest win count (but I don't think this really matters) move b will be
> the winner, again, because its win count is based on more reliable playouts
> that occur later in the game.
> What do you think?
> -Richard
> 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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