▁group , ▁without ▁the ▁inter vent ional ▁element . ▁Thus , ▁when ▁everything ▁else ▁except ▁for ▁one ▁inter vention ▁is ▁held ▁constant , ▁research ers ▁can ▁cert ify ▁with ▁some ▁certain ty ▁that ▁this ▁one ▁element ▁is ▁what ▁caused ▁the ▁observed ▁change . ▁In ▁some ▁instances , ▁having ▁a ▁control ▁group ▁is ▁not ▁eth ical . ▁This ▁is ▁sometimes ▁solved ▁using ▁two ▁different ▁experimental ▁groups . ▁In ▁some ▁cases , ▁independent ▁variables ▁cannot ▁be ▁manip ulated , ▁for ▁example ▁when ▁testing ▁the ▁difference ▁between ▁two ▁groups ▁who ▁have ▁a ▁different ▁disease , ▁or ▁testing ▁the ▁difference ▁between ▁g enders ▁( ob viously ▁variables ▁that ▁would ▁be ▁hard ▁or ▁un eth ical ▁to ▁assign ▁participants ▁to ). ▁In ▁these ▁cases , ▁a ▁quasi - ex perimental ▁design ▁may ▁be ▁used . ▁ ▁C aus al ▁att ribution s ▁In ▁the ▁pure ▁experimental ▁design , ▁the ▁independent ▁( predict or ) ▁variable ▁is ▁manip ulated ▁by ▁the ▁research er ▁- ▁that ▁is ▁- ▁every ▁particip ant ▁of ▁the ▁research ▁is ▁chosen ▁randomly ▁from ▁the ▁population , ▁and ▁each ▁particip ant ▁chosen ▁is ▁assigned ▁randomly ▁to ▁conditions ▁of ▁the ▁independent ▁variable . ▁Only ▁when ▁this ▁is ▁done ▁is ▁it ▁possible ▁to ▁cert ify ▁with ▁high ▁probability ▁that ▁the ▁reason ▁for ▁the ▁differences ▁in ▁the ▁outcome ▁variables ▁are ▁caused ▁by ▁the ▁different ▁conditions . ▁Therefore , ▁research ers ▁should ▁choose ▁the ▁experimental ▁design ▁over ▁other ▁design ▁types ▁whenever ▁possible . ▁However , ▁the ▁nature ▁of ▁the ▁independent ▁variable ▁does ▁not ▁always ▁allow ▁for ▁manip ulation . ▁In