" ▁for ▁the ▁price ▁of ▁an ▁hour ▁of ▁work ▁( this ▁is ▁often ▁referred ▁to ▁in ▁Marx ian ▁econom ics ▁as ▁the ▁" mon et ary ▁equivalent ▁of ▁labour ▁time ", ▁or ▁ME LT ) ▁and ▁one ▁can ▁discuss ▁the ▁extent ▁to ▁which ▁labour ▁is ▁under valu ed ▁or ▁over valu ed ▁in ▁a ▁relative ▁( com par ative ) ▁sense . ▁This ▁interpretation ▁is ▁not ▁accepted ▁by ▁all ▁Marx ist ▁sch ol ars , ▁because — the ▁critics ▁argue — all ▁price - value ▁differential s ▁among ▁different ▁outputs ▁are ▁necessarily ▁and ▁by ▁definition ▁cancel led ▁out ▁at ▁the ▁aggregate ▁level , ▁not ▁just ▁in ▁an ▁assumed ▁theoretical ▁model , ▁but ▁in ▁reality . ▁They ▁point ▁to ▁pass ages ▁where ▁Marx ▁suggests ▁that ▁the ▁sum ▁of ▁product - values ▁must ▁be ▁equal ▁to ▁the ▁sum ▁of ▁production ▁prices , ▁imp lying ▁that ▁there ▁can ▁never ▁be ▁more ▁new ▁product - value ▁or ▁less ▁product - value ▁than ▁is ▁expressed ▁by ▁the ▁sum ▁total ▁of ▁output ▁production ▁prices . ▁If ▁they ▁argue ▁that ▁equality ▁does ▁not ▁hold , ▁then ▁there ▁cannot ▁be ▁any ▁determin ate ▁quant itative ▁relationship ▁between ▁production ▁prices ▁and ▁product - values . ▁ ▁Product - values ▁in ▁Marx ' s ▁sense ▁quite ▁simply ▁cannot ▁be ▁directly ▁observed , ▁only ▁infer red ▁from ▁the ▁actual ▁behaviour ▁of ▁trad ing ▁relations . ▁In ▁that ▁regard , ▁Marx ' s ▁concept ▁of ▁"" value " ▁has ▁exactly ▁the ▁same ▁status ▁as ▁the ▁official ▁category ▁of ▁" value ▁added " ▁( which ▁is