It is almost impossible to say, except for concrete situations, to what extent irregularity of wage rates is due to differences in individual efficiency and to what extent to other causes. Such factors as differences in bargaining power, differences in the policy or efficiency of the employers, slight differences in the character of the work performed, local differences in the supply and demand situation for the type of labor in question, and the like, certainly account for a great many of the irregularities. Prof. Marshall has expressed one view of the matter well. He writes, "Cliffe Leslie and some other writers have naively laid stress on local variations of wages as tending to prove that there is little mobility among the working classes, and that competition among them for employment is ineffective. But most of the facts they quote ... are only half facts and when the missing halves are supplied, they generally support the opposite inference to that on behalf of which they are quoted."[78] In R. H. Tawney's study of "Minimum Rates in the Tailoring Industry" (Great Britain) a vigorous statement of the opposite view is given. He writes, "The wages paid to a group of workers in a given industry and a given area depend, in fact, very often not on the conditions obtaining in that industry in other areas, but on the conditions obtaining in that area in other industries."[79] It can be affirmed that the irregularity of wages is due to a considerable extent to other causes than differences in the efficiency of individuals. As D. A. McCabe writes, "Very little seems to be known as to differences of efficiency among men engaged in the same kind of work." But as he adds, "It is safe to assume, however, that they are not reflected in time-working trades with any exactness by the wages paid, even where there is no trade union minimum."[80]