[Bug-gnubg] Odd-Even evals - checkerplay

[Henrik Bukkjaer]

Hi all fellow BG freaks!

Just back from vacation, I'm 400+ mails behind in this forum...

Anyway, I have also noticed the odd-even ply evaluation variations that
have been discussed, but unfortunately I have not had the time to
store/analyze the findings. I will start to collect them from now on.

My question:
After having discussed the problem, do you have any explanation why the
systematic variance occurs?

It has been suggested here to use "half-plies" to correct the problem (to
give sort of an interpolation).
To me this seems wrong - even if it would give more accurate evaluations.
It is like treating the symptoms as opposed to find the root cause and
completely cure the gnu.

Also it has been suggested that the odd-even problem is not important for
checkerplay but only for cube decisions.
This is not correct.
The odd-even problem only applies to some kind of positions. For some
checkerplay decisions you must decide between two checkerplays leading to
very different gameplans. The odd-even problem could apply to only one of
these, resulting in gnu choosing play A on 0-, 2- and 4-ply, but choosing
play B on 1- and 3-ply evaluation.

I don't have a clear cut example of this at hand, but I have seen it
happen.

However I have an example from a match I played yesterday, where the
pattern occur - it just didn't tip the scales:

Pos: zG2DARjYm4URIA
Match: UQkpAgAAIAAA
(I was using 0.13b on the Ortega & Kleinman 2001 ME table)

The position shows a very interesting checkerplay decision - go for the
prime (13/7 9/7) or go for the stronger board and fewer blots (9/3 5/3).
(The roll of 6-2 leads to the same checkerplays as the actual roll of 2-2).

Gnu chooses the prime move on all plies. I have not yet rolled out the
position.

Ply and Difference (in ME):
0:  0.17%
1:  0.81%
2:  0.22%
3:  1,10%
4:  0.54%

The difference in wins is less than 2% (58.0% vs 56.2%) at 2-ply, but gets
as high as 5+% on 3-ply (60.1% vs. 54.9%)

Best regards,
Henrik Bukkjaer