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.
Don On Sat, Jun 22, 2013 at 3:03 PM, Richard Lorentz <[email protected]> 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. >> >> ______________________________**_________________ >> Computer-go mailing list >> [email protected] >> http://dvandva.org/cgi-bin/**mailman/listinfo/computer-go<http://dvandva.org/cgi-bin/mailman/listinfo/computer-go> >> >> > ______________________________**_________________ > Computer-go mailing list > [email protected] > http://dvandva.org/cgi-bin/**mailman/listinfo/computer-go<http://dvandva.org/cgi-bin/mailman/listinfo/computer-go> >
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