Author: Knut Bjørnar Wålberg
Date: 11:06:42 09/09/03
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Your friend remembers correctly, there is something mentioned in the T-notes from June 29th 2003 (http://www.chessbase.com/support/support.asp?pid=271): -"Skip even plies" changes this behavior somewhat. If you set the dialogue for "9" through "13" and check this box, you'll get just three games instead of five: it will play games using search depths of nine, eleven, and thirteen plies -- skipping over "10" and "12". There's a reason for using this setting: some chess engines (particularly older ones) become somewhat tactically "blind" at even-ply search depths (they fail to consider responses to surprise moves by the opponent and neglect defense a bit). So you'll tend to get better play with these older engines if you check the "Skip even plies" box. And I found a brief explanation (www.cs.ualberta.ca/~games/articles/aphidacc.ps.gz): -Many game-tree search programs exhibit an effect based on the parity of the search depth (odd or even number of ply). Scores are stable when you look at results from the odd plies only, or even plies only, but are sometimes unstable when you mix the two. Thus, we use the largest ply value with the same parity, instead of always using the largest ply value available. ...which makes sense. Then finally a paper by among others Aske Plaat (http://www.cs.vu.nl/~aske/Papers/optim.pdf): -An interesting feature is that all three programs, Othello and chess in particular,have significantly worse performance for even depths. The reason for this can be seenif we look at the structure of the minimal tree. In going from an odd to an even ply,most of the new nodes are nodes where a cutoff is expected to occur. For the minimalgraph, their children count as just one node access. However, the search algorithmmay have to consider a number of alternatives before it finds one that causes the cutoff.Therefore, at even plies, move ordering is critical to performance. On the other hand,in going from an even to an odd ply, most of the new nodes are children of nodes whereno cutoff is expected. All of the children are part of the minimal graph. Hence, at thesenodes move ordering has no effect since all children have to be searched anyway.The preceding leads to an important point: reporting the efficiency of a fixed-depthsearch algorithm based on odd-ply data is misleading. The odd-ply iterations givean inflated view of the search efficiency. For odd-ply searches, all three programs are searching with an efficiency similar to the results reported for other programs.However, the even-ply data is more representative of real program performance and,on this measure, it appears that there is still room for improvement. In light of this, theHitech results of 1.5 for 8-ply searches seem even more impressive [3]. The explanation above seems ok, but I'm not among the most qualified on these boards to comment. Oh, BTW, all quotes found by the help of Google. :)
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