“Except for rough and popular purposes, any measurement of mental capacity in terms of age is unsatisfactory.... The unit fluctuates in its significance all along the scale. When the child is just beginning to walk and talk, when he is 7 or 8, when he is 10 to 11, when he is on the verge of puberty—at these different periods a retardation of a single year means very different things” (_164_, p. 36). A number of good psychologists including Yerkes, Terman, and Kuhlmann, agree with Burt in maintaining that a year of retardation at different ages has very different significance. With this statement of Burt, Pearson takes issue, saying: “Can the psychologist to the London County Council ever have seen the growth curves of children, or would he write thus?... There is no valid reason to suppose that a year's growth in mental power may not be taken for all practical purposes to mean the same unit for ages of 6 to 15, the period for which Binet and Jaederholm have used the tests” (_164_, p. 44). Like many other apparently opposite statements both contain truth. The conflict arises apparently, first from a disagreement between the data obtained with the Jaederholm form of the scale, on which Pearson bases his statement, and data obtained with other forms of the scale; second, from a discrepancy in the points of view. Pearson stresses the fact that the mental year-marks equal average growth increment with the Jaederholm scale (_167_). He shows that the regression of years of mental excess (or deficiency) on increase of life-age is a straight line, just as he found it with physical measurements. Moreover, the standard deviation of the mental measurements for the entire group of normal school children, 6-14 years of age, was found to be about one year of mental age (.96 year for the corrected data) (_167_). To which Pearson's opponents might reply, these facts are of comparatively little significance unless the _deviations for the separate ages_ are alike in terms of these year units on the scale. Neither linear regression nor the balancing of years of excess by years of deficiency at each age indicates that the deviations of the separate ages are alike in terms of the year units. The new Stanford scale, for example, shows both of these conditions and yet the range of months of life-ages which sets off the middle 50% of the children of the different tested ages increased decidedly from 6 to 14 years of age. The middle half of the tested ages, for example, at age VI on the scale include a randomly selected group of six-year-old children whose range of life-age is ten months, at age VIII on the scale this range is 13.4 months, at X it is 16 months, at XII, 20 months, and at XIV, 26 months. “The number of 6-year-old children testing 'at age' is approximately twice as great as the number of 12-year-olds testing at age, and 50% greater than in the case of the 9-year-olds” (_196_, p. 557).