Author: Chrilly Donninger
Date: 23:35:39 12/14/05
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 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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