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Why do bots have an even/odd effect?

Posted By: Tom Keith
Date: Wednesday, 12 August 2009, at 2:11 p.m.

In Response To: Plies again (playBunny)

Bot evaluation functions are typically set up with no input for "which side is on roll". Instead, the evaluation function assumes one side (say white) is on roll. When is it's black's turn, the bot simply inverts the colors of all the checkers (so that "white" is now on roll), feeds that to the evaluation function, and reverses the result that comes back.

This leads to an even/odd effect between levels of lookahead. Let's say it is white's turn. A 0-ply evaluation looks at the board from white's perspective. But a 1-ply evaluation looks ahead one roll before evaluating the position. So all the evaluations at 1-ply are from black's perspective.

Any bot that doesn't have an input for "which side is on roll" has at least a slight even/odd effect. But GnuBG's is quite a bit larger than most. Here's why I think that is:

(1) GnuBG was trained to make its 0-ply evaluation reproduce as closely as possible the results of it's 2-ply evaluation. This gives accurate relative evaluations, which is what you want for good checker play. But it does little to force accurate absolute equities. In particular, it does little to force 0-ply's equity to mimic 1-ply.

(2) GnuBG was specifically trained to give accurate absolute equities in cube oriented situations. By "cube oriented", I mean any position in which the side on roll is considering doubling. It was trained for these types of positions to the exclusion of all other positions with respect to absolute equity.

In other words, GnuBG is trying to squeeze as much as possible out of its NN. It has good relative equities everywhere and good absolute equities in cube oriented positions.

While the even-odd effect might seem a bit disconcerting, it is not necessarily a bad thing. As long as the bot plays well, and gives accurate cube decisions, that's what is important.

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