The comparison of the year units on the Binet scale with the diagrams in Fig. 3 shows that if the scale at each life-age shut out the same lowest proportion, say half, of the children of that age, then the year units might be regarded as equal in the sense of equal average growth increments, as Pearson suggests. A child 7 years of age testing VII would be at least one annual average-growth unit higher in mental development than one of 6 years testing VI, and so with each age until the limit of development had been reached. This is the condition approximated closely for children by the new Stanford scale and the corrected Jaederholm data. Since there is little prospect, however, even with a scale perfected so far as its age norms are concerned, that the total distributions for each of the different years would be the same multiple of the year-units, the main significance of the age units is in permitting the statement that a child had reached the tested development normal for the children of a certain age. It is also legitimate to use years of retardation as a short way of expressing rough borderlines when they happen thus to afford an easy method of empirically describing equivalent borderlines for a particular scale. This is what I have done for convenience in Part One of this book. I certainly do not mean to contend that four-years retardation has theoretically the same significance at different ages, in terms of the deviation of the separate ages. To me the Binet years are no more than names for certain positions on the scale. To most psychologists who have been dealing with the measurement of mental development, I believe that the most significant concept of equivalent units would be in terms of the deviations for each age provided that the form of the distributions remained normal. But the deviations vary so much in the terms of the year units that it is not likely that they will be willing to accept a _year of excess or deficiency_ as an equivalent unit for different ages with the common forms of the scale in use in English-speaking countries. Moreover, below the age of 6 and above 15, the limits which Pearson discusses, there is good reason to expect the year unit to vary still further. This Pearson recognizes for the complete developmental curve. It is only at the intermediate years, in which the average increases are most constant in relation to the deviations of the separate ages, that the year unit may be at all serviceable in measuring the deviation of a child from the norm of his age.