▁the ▁majority ▁of ▁cases , ▁the ▁expert ' s ▁personal ▁relation ▁to ▁the ▁def endant ▁is ▁considered ▁and ▁irrelevant . ▁ ▁The ▁trib unal ▁itself , ▁or ▁the ▁judge , ▁can ▁in ▁some ▁systems ▁call ▁upon ▁exper ts ▁to ▁techn ically ▁evaluate ▁a ▁certain ▁fact ▁or ▁action , ▁in ▁order ▁to ▁provide ▁the ▁court ▁with ▁a ▁complete ▁knowledge ▁on ▁the ▁fact / action ▁it ▁is ▁jud ging . ▁The ▁expert ise ▁has ▁the ▁legal ▁value ▁of ▁an ▁ac quisition ▁of ▁data . ▁The ▁results ▁of ▁these ▁exper ts ▁are ▁then ▁compared ▁to ▁those ▁by ▁the ▁exper ts ▁of ▁the ▁parties . ▁ ▁The ▁expert ▁has ▁a ▁great ▁responsibility , ▁and ▁especially ▁in ▁pen al ▁tri als , ▁and ▁per j ury ▁by ▁an ▁expert ▁is ▁a ▁sever ely ▁pun ished ▁crime ▁in ▁most ▁countries . ▁The ▁use ▁of ▁expert ▁witness es ▁is ▁sometimes ▁critic ized ▁in ▁the ▁United ▁States ▁because ▁in ▁civil ▁tri als , ▁they ▁are ▁often ▁used ▁by ▁both ▁sides ▁to ▁advoc ate ▁differ ing ▁positions , ▁and ▁it ▁is ▁left ▁up ▁to ▁a ▁j ury ▁to ▁decide ▁which ▁expert ▁witness ▁to ▁believe . ▁Although ▁exper ts ▁are ▁leg ally ▁prohib ited ▁from ▁express ing ▁their ▁opinion ▁of ▁submitted ▁evidence ▁until ▁after ▁they ▁are ▁h ired , ▁sometimes ▁a ▁party ▁can ▁sur m ise ▁before hand , ▁because ▁of ▁reputation ▁or ▁prior ▁cases , ▁that ▁the ▁testim ony ▁will ▁be ▁favor able ▁regardless ▁of ▁any ▁basis ▁in ▁the ▁submitted ▁data ; ▁such ▁exper ts ▁are ▁commonly ▁dispar aged ▁as ▁" h