CH. 4 THE CHOICE OF UNITS 45

pected) user cost corresponding to a level of employ-ment N,.

Thus in the case of each homogeneous commodity,for which Or= ψr(Nr) has a definite meaning, we canevaluate Zr= ψr(Nr) in the ordinary way; but wecan then aggregate the Nr’s ina way in which we cannotaggregate the O,’s, since ΣO, is not a numerical quan-tity. Moreover, if we can assume that, in a givenenvironment, a given aggregate employment will bedistributed in a unique way between different indus-tries, so that N, is a function of N, further simplifica-tions are possible.