# [Computer-go] Computer-go Digest, Vol 54, Issue 4

Greg Schmidt gschmidt958 at yahoo.com
Thu Jul 3 06:18:32 PDT 2014

```My original example was unrealistic and on the extreme side to make a point.  However if there are nodes with say 7/10, 12/20, and 50/100 how should they be ranked?  In some sense, the first one seems promising since we've only searched just a few nodes, yet we are mainly seeing wins (granted, the last one expresses more confidence).  To me, this would seem like an argument that favors win rate over win count (and we don't always have the benefit of extending the search for these "low confidence nodes" until the law of large numbers sorts things out).

Perhaps there is an argument that the UCB formula "won't generally let this happen" since it takes into consideration both win rate and tries to increase confidence by promoting the visit of nodes with low visit counts.  Still, I can envision cases where time is running out, and UCT has just recently discovered a promising new branch.

If the choice were 1/2 vs. 50/100, the second expresses more confidence yet one could argue we should search the first move more (given that time is available to do so) to find out if it is, in fact, better than .5 (since being low confidence, it has a reasonable chance of being better than the 2nd).  So the general question becomes how to effectively trade off win rate with win rate confidence?

FWIW, the sample code at
http://mcts.ai/code/python.html

Does something completely different, it selects the root node which has the most number of visits:

"return sorted(rootnode.childNodes, key = lambda c: c.visits)[-1].move # return the move that was most visited"

On Thu, Jul 03, 2014 at 10:57:17AM +0200, Stefan Kaitschick
wrote:
> On Thu, Jul 3, 2014 at 9:00 AM, Darren Cook <darren at dcook.org>
wrote:
>
> >
> > If you had a choice between a 1% 65,000-wins move
and a 70% 7-wins move,
> > MCTS will keep exploring the 70% move, until it
either reaches 65,001
> > wins, and can be chosen, or the winning percentage
comes down to 1% also.
> >
> > BTW, that implies it would be very difficult to
ever reach the situation
> > you describe, as 1% win rate moves wouldn't be
given 650,000 trials
> > (unless all other moves on the board are equally
> > clearly lost).
> >
>
> What I dont understand, is why the variation that's
trying to catch up has
> to absolutely overtake the leader.
> Shouldn't there be a substantial bonus for a late high
success rate?

The way this bonus is often implemented is that if a
disparity between
the move with the highest winrate and the move with the
highest number
of simulations is different, the program spends some extra
time
searching to give a chance to resolve this.

(Conversely, if some move has been simulated so much
more that no
other move can overtake it in the number of simulations in
the alloted
time anymore, the search can be stopped early.)

P.S.: No, I don't like Japanese byoyomi for Pachi. ;-)

--

Petr Baudis
Life is short, the craft long,
opportunity fleeting, experiment
treacherous, judgment difficult.  --
Hippocrates

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End of Computer-go Digest, Vol 54, Issue 4
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