Previously, we have emphasized the distinction between the concepts of productivity (DMVP) and work (pp. 4.8:17-9); indeed, the conflation of these two concepts is one of many errors stemming from a cost theory of value. For this reason, the "equal pay for equal productivity" principle, promoted by free-market competition, should not be confused with the popular slogan of "equal pay for equal work." The measurement of productivity is a highly sophisticated procedure, encompassing many factors not included in the narrower concept of work. First, productivity is directly affected not only by the amount of time and effort expended, but also by the skills of the particular worker. For instance, in one hour of labor, Friday may easily be able to weave twice as many palm leaves into thatch as Crusoe, even if Crusoe works at his fastest possible pace. Within any given labor market, at any moment in time, there will typically be statistical variations in the distribution of such skills among different classes of individuals in a population. Class-C workers, as a consequence, may exhibit either above-average or below-average abilities in specific tasks. In the free market, such differences give rise to observable mathematical disparities in hiring percentages and average wages among class C and other classes of workers.