Author: Stephen Ham
Date: 08:38:35 03/02/04
Go up one level in this thread
On March 02, 2004 at 10:29:40, martin fierz wrote: >On March 02, 2004 at 10:08:15, Stephen Ham wrote: > >>On March 02, 2004 at 05:19:52, martin fierz wrote: >> >>>On March 01, 2004 at 14:40:29, Stephen Ham wrote: >>> >>>>On March 01, 2004 at 14:06:15, William Penn wrote: >>>> >>>>>Due to a unique bug in Shredder 7 and Shredder 8, its analysis is unreliable. >>>>>Only the first move can be trusted. Forget the rest! Sometimes it's OK, but >>>>>often it's garbage, or somewhere in-between. If you want reliable analysis, >>>>>choose a different engine (not Shredder). >>>>>WP >>>> >>>>Dear William, >>>> >>>>I suspect that you misread my post. Yes, in Shredder 7 & 8, only the PV is >>>>reliable. This is true for some other engines too. For example, I noticed this >>>>in my matches with Friz 6a and Nimzo 7.32. >>>>(http://www.correspondencechess.com/campbell/ham/ham.htm). But the a-pawn losing >>>>move that was second in move-ordering was the PV of the second line of the move >>>>ordering sequence. >>>> >>>>All the best, >>>>Stephen >>> >>>hi stephen >>> >>>while i understand what you want to say, you are not saying it correctly. >>>the PV is the whole line given by shredder. the first move of the PV is always >>>correct, the entire rest of the PV *can* be garbage but must not. >>> >>>so your statemtent "only the PV is reliable" is totally wrong, but i think >>>that's because you use PV in the sense of "first move of PV" - but since you're >>>the only one who does this, it will lead to confusion :-) >>> >>>cheers >>> martin >> >>Hi Martin, >> >>Sorry for my poor communications. My ignorance of chess engine vernacular is >>showing. Since I'm in finance professionally, PV has a meaning for me which I >>assumed to be similar to the data output in chess engines. Therefore it may be >>best to offer an illustration of what I meant. >> >> >>27 Re1 blah blah blah blah blah uzw... +1.09 16-ply depth >> >>What I meant by PV was merely the 1-ply that is represented by 27 Re1, since the >>remaining thread in S7 and S8 is highly suspect. > >right, the first move of the PV is correct, the rest of the PV is suspect - in >shredder, but also in some other programs; those which take the PV from the >hashtable. PV stands for "principal variation", so it's the entire variation >given above, Re1 including the whole blahblah. > > >>However, after move ordering through 16-ply depth made 27 Re1 the prime >>candidate to be played, it then picked 27 a3 to be the next move analyzed in the >>move ordering sequence. It's my understanding that this move ordering process >>allocates greater time for analysis of the best candidates. > >well.... i don't think so! it depends a bit on what you're doing. if you are >using shredder in multi-variation mode, then this is correct. i.e. if you have >it display the best N moves, then the best N moves are ordered by value, and it >computes an exact value for them. keep in mind that normally, programs only >compute a value for the best move, and prove that all others are worse. >therefore, root move ordering doesn't usually mean that the second move in the >list is the second best; simply because the program has no idea which of the >remaining N-1 moves has the second best value - it only knows that all of them >have a lower (or equal) value than the best move. >one common technique is to order the remaining N-1 moves according to the number >of nodes it took to "refute" them, i.e. to prove that they are not better than >the current best move. the idea being that you need very few nodes to prove that >a really crappy move is in fact crappy. but still, this doesn't order moves >according to value. > >so, if you were not using "display N best moves" with N>1, the 27.a3 move is not >second best just because it's searched as second move. > >>So while S8 still displayed 27 Re1 as the best move, its analysis next switched >>to 27 a3 blah blah blah blah blah uzw. It's the 27 a3 move that just drops a >>pawn, so I was shocked to see it placed second in the move ordering sequence. > >for this not to shock you, you must understand that it doesn't really matter as >long as Re1 is really the best move. then you must search all remaining moves, >and the order in which you search them is irrelevant. >the only time this gets relevant is when Re1 is in fact not the best move, and >your time is running out, and you only manage to search a couple of the >remaining moves after searching Re1, and you would have found that 27. XY is >better than Re1 but unfortunately you searched 27. a3 and other crap first and >wasted your time, and didn't find the better move when you decided it was time >to play now. > >my personal experience with computers and deep analysis is that all automated >analysis is inferior to human-guided analysis. let the machine think a bit, in N >best moves mode, see whether you agree and move forward along a sensible line. >the "deep position analysis" feature of the fritz family mimics this behavior, >but i don't like it too much. i have seen fritz look at really weird moves in >it's deep position analysis feature, which i think you could confidently skip as >a human; so i would do it all interactively. > >cheers > martin Dear Martin, Thanks for your highly educational post. I always assumed that PV meant "present value", as it does in Finance (my mistake). So I mistook PV to be akin to the "root" of the analysis tree. Also, I've learned that chess engines are more likely to find the strongest move only when displaying the PV. Therefore I only had it do that (rather than having it display the top 3-lines, for example). As for move ordering, I've noticed that (all things being equal) the second move in the ordering sequence virually always gets more calculation time than the 10th, which in turn gets more than the 20th. So I assumed that this was the engines way to devote more calculation time to meaningful candidates and less time to moves that are probably weak. But you're now telling me that this is incorrect, Martin. So then, why are the lines placed closest to the PV in the move ordering process given generally more time? For the record, as a CC player, I find my own moves and don't use engines to generate my moves. Therefore I don't have much interest in DPA other than as a curiosity. Instead, my enjoyment comes in subsequently checking my games against engines to see which are most likely to find my moves, or to show me my errors. I tend to play in a rather technical/positional style, so engines often have difficulty finding moves that are related to long-range planning, versus winning pawns, etc. This is time consuming so I admit to having only conducted a couple dozen tests. An example of these tests is seen in my review of S7 at ChessCafe.com. So far, all of my tests were conducted in Infinite Analysis mode. Now I'm wondering if I should have instead conducted my analysis in DPA mode. Any thoughts, Martin? Thanks again for your valuable input. All the best, Stephen
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