Hi Remi,
I understood this. What I find strange is that using -1/1 should be
equivalent to using 0/1, but your algorithm behaves differently: it
ignores lost games with 0/1, and uses them with -1/1.
Imagine you add a big constant to z. One million, say. This does not
change the problem. You get either 1000000 or 1000001 as outcome of
a playout. But then, your estimate of the gradient becomes complete
noise.
So maybe using -1/1 is better than 0/1 ? Since your algorithm
depends so much on the definition of the reward, there must be an
optimal way to set the reward. Or there must a better way to define
an algorithm that would not depend on an offset in the reward.
There is still something wrong that I don't understand. There may be
a way to quantify the amount of noise in the unbiased gradient
estimate, and it would depend on the average reward. Probably
setting the average reward to zero is what would minimize noise in
the gradient estimate. This is just an intuitive guess.
Okay, now I understand your point :-) It's a good question - and I
think you're right. In REINFORCE any baseline can be subtracted from
the reward, without affecting the expected gradient, but possibly
reducing its variance. The baseline leading to the best estimate is
indeed the average reward. So it should be the case that {-1,+1}
would estimate the gradient g more efficiently than {0,1}, assuming
that we see similar numbers of black wins as white wins across the
training set.
So to answer your question, we can safely modify the algorithm to use
(z-b) instead of z, where b is the average reward. This would then
make the {0,1} and {-1,+1} cases equivalent (with appropriate scaling
of step-size). I don't think this would have affected the results we
presented (because all of the learning algorithms converged anyway, at
least approximately, during training) but it could be an important
modification for larger boards.
-Dave
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