▁requirements ▁of ▁experimental ▁research ▁designs ▁is ▁the ▁necessity ▁of ▁elimin ating ▁the ▁effects ▁of ▁sp ur ious , ▁interven ing , ▁and ▁ante ced ent ▁variables . ▁In ▁the ▁most ▁basic ▁model , ▁cause ▁( X ) ▁leads ▁to ▁effect ▁( Y ). ▁But ▁there ▁could ▁be ▁a ▁third ▁variable ▁( Z ) ▁that ▁influ ences ▁( Y ), ▁and ▁X ▁might ▁not ▁be ▁the ▁true ▁cause ▁at ▁all . ▁Z ▁is ▁said ▁to ▁be ▁a ▁sp ur ious ▁variable ▁and ▁must ▁be ▁controlled ▁for . ▁The ▁same ▁is ▁true ▁for ▁interven ing ▁variables ▁( a ▁variable ▁in ▁between ▁the ▁supposed ▁cause ▁( X ) ▁and ▁the ▁effect ▁( Y )), ▁and ▁ante ced ing ▁variables ▁( a ▁variable ▁prior ▁to ▁the ▁supposed ▁cause ▁( X ) ▁that ▁is ▁the ▁true ▁cause ). ▁When ▁a ▁third ▁variable ▁is ▁involved ▁and ▁has ▁not ▁been ▁controlled ▁for , ▁the ▁relation ▁is ▁said ▁to ▁be ▁a ▁zero ▁order ▁relationship . ▁In ▁most ▁practical ▁applications ▁of ▁experimental ▁research ▁designs ▁there ▁are ▁several ▁causes ▁( X 1 , ▁X 2 , ▁X 3 ). ▁In ▁most ▁designs , ▁only ▁one ▁of ▁these ▁causes ▁is ▁manip ulated ▁at ▁a ▁time . ▁ ▁Ex perimental ▁designs ▁after ▁Fish er ▁Some ▁efficient ▁designs ▁for ▁estim ating ▁several ▁main ▁effects ▁were ▁found ▁independently ▁and ▁in ▁near ▁succession ▁by ▁Raj ▁Ch andra ▁B ose ▁and ▁K . ▁K ish en ▁in ▁ 1 9 4 0 ▁at ▁the ▁Indian ▁Statist ical ▁Institute , ▁but ▁remained ▁little ▁known ▁until ▁the ▁P lack ett –