ices ▁from ▁product - values ▁according ▁to ▁some ▁mathematical ▁function , ▁among ▁other ▁things ▁because , ▁to ▁find ▁labour - values , ▁a ▁relationship ▁between ▁product - pr ices ▁and ▁labour ▁hours ▁worked ▁must ▁already ▁be ▁assumed . ▁What ▁we ▁can ▁verify ▁is ▁as ▁follows : ▁How ▁systems ▁of ▁exchange ▁have ▁function ed ▁in ▁history . ▁To ▁what ▁extent ▁production - cost s ▁and ▁the ▁r uling ▁profit ▁rates ▁actually ▁determine ▁market ▁prices ▁for ▁products . ▁The ▁relationship ▁between ▁hours ▁worked ▁and ▁outputs ▁produced . ▁Whether ▁the ▁capital ist ▁production ▁system ▁does ▁indeed ▁evol ve ▁histor ically ▁in ▁the ▁way ▁predicted ▁by ▁value ▁theory . ▁ ▁Emp ir ical ▁criticism ▁The ▁empir ical ▁criticism ▁is ▁that ▁Marx ' s ▁law ▁of ▁value ▁contradict s ▁the ▁known ▁facts ▁about ▁the ▁allocation ▁of ▁resources ▁in ▁capital ist ▁society . ▁ ▁The ▁main ▁empir ical ▁criticism ▁is ▁simply ▁that ▁there ▁is ▁no ▁observable ▁quant itative ▁correspond ence ▁at ▁all ▁between ▁changes ▁in ▁relative ▁exp end it ures ▁of ▁labour - time , ▁and ▁changes ▁in ▁relative ▁market ▁prices ▁of ▁products , ▁however ▁measured ▁( the ▁measures ▁are ▁also ▁cont ested , ▁for ▁example ▁on ▁the ▁ground ▁that ▁qual it atively ▁different ▁kinds ▁of ▁labour ▁cannot ▁be ▁compared ▁and ▁equ ated ). ▁Most ▁critics ▁have ▁tried ▁to ▁ref ute ▁Marx ' s ▁theory ▁with ▁a ▁mathematical ▁model , ▁rather ▁than ▁looking ▁at ▁real ▁data ▁to ▁see ▁if ▁the ▁capital ist ▁economy ▁really ▁behav es ▁in ▁the ▁way ▁Marx ▁claims ▁it ▁does . ▁ ▁A ▁recent ▁empir ical