A Cure for Pathological Behavior in Games that Use Minimax

Minimax haJ long been the 4tandard method of eva luating game tree nodeJ in game-playing programJ. The general aJJUmption underlying theu program" i3 that deepening uarch improve" play. Recent work ha8 �hown that thi1 a88umption iJ not alway" valid; for a large clau of gameJ and evaluation function!, deepening search decreauJ the probability of making a correct move. ThiJ phenomena i" called game tree pathology. Two Jtructural propertie3 of game tree" have been suggested aJ cauJeJ of pathology: independence among the value" of Jibling node!, and uniform depth of win node!. Thi! paper ezamine.! the relationship between uniform w1n depth and pathology. .-t game-playing program i" run U3ing two different evaluation function!. The firJt rec07nize4 win! only at the bottom let'el, the ucond at variou3 levels throughout the tree. The min imaz procedure behave" pathologically when the fir�t function i3 u�ed: the ucond .!how! no such pathology. This re�ult con.!titute3 the first experimental et·idence linking uniform win depth to pathology. The effect of not recogni::ing wiM until the bottom of the tree on the probability of making a correct decision j., a lJo analyzed. Thi" analyaia may lead to a general characteri.:ation of pathology in term8 of win di3tribution.