▁about ▁the ▁underlying ▁scientific ▁theory ▁and ▁instrument ▁implementing ▁theory . ▁This ▁witness ▁is ▁an ▁expert ▁witness , ▁called ▁to ▁el ic it ▁opinions ▁that ▁a ▁theory ▁is ▁valid ▁and ▁the ▁instruments ▁involved ▁are ▁reliable . ▁The ▁witness ▁must ▁be ▁qualified ▁as ▁an ▁expert ▁witness , ▁which ▁may ▁require ▁academic ▁qual ifications ▁or ▁specific ▁training . ▁ ▁Report ing ▁witness ▁ ▁Cal led ▁after ▁teaching ▁witness ▁leaves ▁stand . ▁Usually ▁the ▁labor atory ▁techn ician ▁who ▁personally ▁conducted ▁the ▁test . ▁Wit ness ▁will ▁describe ▁both ▁the ▁test ▁and ▁the ▁results . ▁When ▁describing ▁test , ▁will ▁vent ure ▁opinions ▁that ▁proper ▁test ▁procedures ▁were ▁used ▁and ▁that ▁equipment ▁was ▁in ▁good ▁working ▁order . ▁ ▁Non - test ifying ▁exper ts ▁In ▁the ▁U . S ., ▁a ▁party ▁may ▁h ire ▁exper ts ▁to ▁help ▁them ▁evaluate ▁a ▁given ▁case . ▁For ▁example , ▁a ▁car ▁mak er ▁may ▁h ire ▁an ▁experienced ▁mechan ic ▁to ▁decide ▁if ▁its ▁cars ▁were ▁built ▁to ▁specification . ▁This ▁kind ▁of ▁expert ▁opinion ▁will ▁be ▁protected ▁from ▁discovery ▁by ▁the ▁oppos ing ▁party . ▁In ▁other ▁words , ▁if ▁the ▁expert ▁finds ▁evidence ▁against ▁their ▁client , ▁the ▁opposite ▁party ▁will ▁not ▁automatically ▁gain ▁access ▁to ▁it . ▁This ▁privile ge ▁is ▁similar ▁to ▁the ▁work - product ▁doctrine ▁( not ▁to ▁be ▁confused ▁with ▁att orney – client ▁privile ge ). ▁ ▁The ▁non - test ifying ▁expert ▁can ▁be ▁present ▁at ▁the ▁trial ▁or ▁hearing ▁to ▁aid ▁the ▁att orney ▁in ▁asking ▁questions