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[Bug-gnubg] Bug: 1-ply outperforming 2-ply

From: Philippe Michel
Subject: [Bug-gnubg] Bug: 1-ply outperforming 2-ply
Date: Mon, 13 Mar 2017 23:23:14 +0100 (CET)
User-agent: Alpine 2.20 (BSF 67 2015-01-07)

I've tried swapping the players, using different dice generators
(random.org, MT), but the 1-ply player almost ALWAYS beats the 2-ply
player on my machine, by A LOT, even winning by a gammon a fair amount
of the time. Very strange. Shouldn't the 2-ply player be a lot better
than the 1-ply player? I've tried running with & without NN pruning,
move filters, etc, but I always get the same result! What's going on?

In the match attached to your mail, if we analyze it at 1-ply, we get that 1-ply played perfectly (obviously) and 2-ply dropped about 4% winning chances through its few errors. If we analyze it at 2-ply, this is more or less the opposite. Each analysis concludes that its level outplayed the other and was approximately a 52% favorite.

If this match is typical and the numbers are accurate (we can't know with such a superficial analysis, but they look reasonable), hundreds or thousands of trials would be needed to know with any reasonable certainty which side is stronger. For instance, if you played 10 such matches and one side won 8 of them, the chances of it being the strong one is about 2 to 1 only.

The best evidence available, testing the various levels against the Depreli benchmark, is that 1-ply make about twice as many errors as 2-ply.

I'm afraid you simply don't have enough evidence for your claim :-). How many matches did you run ? What were the results ? On the other hand I could check that the 1-ply and 2-ply moves in your log match what would be played on my machine so there is not issue specific to yours.

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