Author: Tony Werten
Date: 01:50:19 12/16/05
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On December 15, 2005 at 02:35:39, Chrilly Donninger wrote: >Its obvious that the evaluation influences the style of a programm. But >interestingly the shape of the search tree has also a significant influence. As >a rule of thumb: The programm prefers lines with larger (sub-)trees. If one >increases a certain extension the chances for such moves to be played increases >(to opposite holds for pruning). >I am certain, that this effect exists, but I do not exactly know the reason. One >explanation is: The evaluation consists of a true term which properly reflects >the value of the position and white noise. If the programm has many choices, >because the search tree is large, the expected value of the white noise is >higher. For non statisticans: If one picks from a bag with 1000 numbers randomly >1 number and the next time 10 numbers, the chances are very high, that the >maximum of the 10 numbers is greater than the number picked first. This seems to be correct. I've once accidently tested this. I forgot to return a result from my evaluation function resulting in basicly returning a random score. Yet, the engine could still solve quite a few (tactical) testpositions. The reason seemed to be that the more mobility one has, the bigger the chance of getting a better score. Winning pieces reduces the opponents mobility, so winning pieces is still good. Tony > >This effect takes of course only place, if the true values of moves are similar. >In this case the move with the highest white noise is choosen. > >One example for the practical consequences can be found in Rybka: Probably a lot >of users think, when they choose the personality "very positionally", that the >programm has - in contrast to "very tactically" - more chess knowledge, that >there is a trade-off between knowledge and search-speed. >In fact the 4 personality settings change 2 numbers. These numbers influence >only the pruning/extension mechanism of the search tree. The "very tactical" >settings prune less than the "very positional" one. I have not tested the >differences in the playing styles, but from the theoretical considerations it >should indeed influence the style. The terms "tactical, positional" are just >labels. One has to give it a name. >The same is probably done in all other programms. E.g. an old Nimzo-version of >mine had already such a setting. The personalities were called aggressive, >solid.. These settings changed the shape of the tree in a similar way. Maybe >Rybka is a Nimzo clone :-) > >Chrilly
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