▁some ▁variable ▁does ▁not ▁enter ▁into ▁the ▁equation , ▁while ▁it ▁does ▁enter ▁the ▁other ▁equation . ▁ ▁A ▁situation ▁in ▁which ▁both ▁the ▁supply ▁and ▁the ▁demand ▁equation ▁are ▁identified ▁ar ises ▁if ▁there ▁is ▁not ▁only ▁a ▁variable ▁Z ▁entering ▁the ▁demand ▁equation ▁but ▁not ▁the ▁supply ▁equation , ▁but ▁also ▁a ▁variable ▁X ▁entering ▁the ▁supply ▁equation ▁but ▁not ▁the ▁demand ▁equation : ▁▁ ▁supply : ▁    ▁▁▁ ▁demand : ▁  ▁▁ ▁with ▁positive ▁b S ▁and ▁negative ▁b D . ▁Here ▁both ▁equations ▁are ▁identified ▁if ▁c ▁and ▁d ▁are ▁non zero . ▁ ▁Note ▁that ▁this ▁is ▁the ▁struct ural ▁form ▁of ▁the ▁model , ▁showing ▁the ▁relations ▁between ▁the ▁Q ▁and ▁P . ▁The ▁reduced ▁form ▁however ▁can ▁be ▁identified ▁easily . ▁ ▁Est imation ▁methods ▁and ▁dist urban ces ▁" It ▁is ▁important ▁to ▁note ▁that ▁the ▁problem ▁is ▁not ▁one ▁of ▁the ▁appropri aten ess ▁of ▁a ▁particular ▁estimation ▁technique . ▁In ▁the ▁situation ▁described ▁[ without ▁the ▁Z ▁variable ], ▁there ▁clearly ▁exists ▁no ▁way ▁using ▁any ▁technique ▁what so ever ▁in ▁which ▁the ▁true ▁demand ▁( or ▁supply ) ▁curve ▁can ▁be ▁estimated . ▁Nor , ▁indeed , ▁is ▁the ▁problem ▁here ▁one ▁of ▁statistical ▁inference — of ▁separ ating ▁out ▁the ▁effects ▁of ▁random ▁disturb ance . ▁There ▁is ▁no ▁disturb ance ▁in ▁this ▁model ▁[...] ▁It ▁is ▁the ▁logic ▁of ▁the ▁supply - dem and ▁equilibrium ▁itself ▁which ▁leads ▁to ▁the ▁difficulty ." ▁( F isher ▁ 1 9 6