▁the ▁diagram , ▁Wright ▁( 1 9 3 4 ) ▁proposed ▁a ▁simple ▁set ▁of ▁path ▁trac ing ▁rules , ▁for ▁calculating ▁the ▁correlation ▁between ▁two ▁variables . ▁The ▁correlation ▁is ▁equal ▁to ▁the ▁sum ▁of ▁the ▁contribution ▁of ▁all ▁the ▁path ways ▁through ▁which ▁the ▁two ▁variables ▁are ▁connected . ▁The ▁strength ▁of ▁each ▁of ▁these ▁contrib uting ▁path ways ▁is ▁calculated ▁as ▁the ▁product ▁of ▁the ▁path - co efficient s ▁along ▁that ▁path way . ▁ ▁The ▁rules ▁for ▁path ▁trac ing ▁are : ▁ ▁You ▁can ▁trace ▁back ward ▁up ▁an ▁arrow ▁and ▁then ▁forward ▁along ▁the ▁next , ▁or ▁for wards ▁from ▁one ▁variable ▁to ▁the ▁other , ▁but ▁never ▁forward ▁and ▁then ▁back . ▁Another ▁way ▁to ▁think ▁of ▁this ▁rule ▁is ▁that ▁you ▁can ▁never ▁pass ▁out ▁of ▁one ▁arrow ▁head ▁and ▁into ▁another ▁arrow head : ▁heads - t ails , ▁or ▁t ails - head s , ▁not ▁heads - head s . ▁ ▁You ▁can ▁pass ▁through ▁each ▁variable ▁only ▁once ▁in ▁a ▁given ▁chain ▁of ▁paths . ▁ ▁No ▁more ▁than ▁one ▁bi - direction al ▁arrow ▁can ▁be ▁included ▁in ▁each ▁path - chain . ▁ ▁Again , ▁the ▁expected ▁correlation ▁due ▁to ▁each ▁chain ▁trac ed ▁between ▁two ▁variables ▁is ▁the ▁product ▁of ▁the ▁standard ized ▁path ▁coefficients , ▁and ▁the ▁total ▁expected ▁correlation ▁between ▁two ▁variables ▁is ▁the ▁sum ▁of ▁these ▁contrib uting ▁path - ch ains . ▁ ▁N B : ▁Wright ' s ▁rules ▁assume ▁a ▁model