▁those ▁cases , ▁research ers ▁must ▁be ▁aware ▁of ▁not ▁cert ifying ▁about ▁caus al ▁att ribution ▁when ▁their ▁design ▁doesn ' t ▁allow ▁for ▁it . ▁For ▁example , ▁in ▁observ ational ▁designs , ▁participants ▁are ▁not ▁assigned ▁randomly ▁to ▁conditions , ▁and ▁so ▁if ▁there ▁are ▁differences ▁found ▁in ▁outcome ▁variables ▁between ▁conditions , ▁it ▁is ▁likely ▁that ▁there ▁is ▁something ▁other ▁than ▁the ▁differences ▁between ▁the ▁conditions ▁that ▁causes ▁the ▁differences ▁in ▁out comes , ▁that ▁is ▁- ▁a ▁third ▁variable . ▁The ▁same ▁goes ▁for ▁studies ▁with ▁correl ational ▁design . ▁( Ad ér ▁& ▁M ellen berg h , ▁ 2 0 0 8 ). ▁ ▁Statist ical ▁control ▁It ▁is ▁best ▁that ▁a ▁process ▁be ▁in ▁reasonable ▁statistical ▁control ▁prior ▁to ▁conduct ing ▁designed ▁experiments . ▁ ▁When ▁this ▁is ▁not ▁possible , ▁proper ▁blocking , ▁rep lication , ▁and ▁random ization ▁allow ▁for ▁the ▁careful ▁conduct ▁of ▁designed ▁experiments . ▁To ▁control ▁for ▁n uis ance ▁variables , ▁research ers ▁inst itute ▁control ▁checks ▁as ▁additional ▁measures . ▁ ▁Investig ators ▁should ▁ensure ▁that ▁un control led ▁influ ences ▁( e . g ., ▁source ▁cred ibility ▁per ception ) ▁do ▁not ▁ske w ▁the ▁find ings ▁of ▁the ▁study . ▁ ▁A ▁manip ulation ▁check ▁is ▁one ▁example ▁of ▁a ▁control ▁check . ▁ ▁Man ip ulation ▁checks ▁allow ▁investig ators ▁to ▁isol ate ▁the ▁chief ▁variables ▁to ▁strength en ▁support ▁that ▁these ▁variables ▁are ▁operating ▁as ▁planned . ▁ ▁One ▁of ▁the ▁most ▁important