Before leaving the question of the significance of units on a scale described in terms of the standard deviation we should ask whether tested mental abilities have been found to distribute normally, _i. e._, in the form of the symmetrical Gaussian curve with each extreme the same distance from the middle measurement. Contrary to the usual supposition in this matter, it seems as if the evidence was somewhat against this assumption, although neither position can be asserted at all dogmatically on the basis of our present data. A résumé of this evidence which I have given below makes it appear that the assumption of a normal distribution will not conflict with a practical use of normal probability tables for medium degrees of ability, but may quite seriously interfere with such use for the borderline of deficiency. There is little doubt, as Pearson believes, that the bulk of the children now in special classes for the retarded in the public schools would fall within the lower range of a normal distribution fitted to the general population. On the other hand, there is likely to be a respectable minority of the deficients which will be beyond such a normal curve. These facts are sufficiently evident, I believe, to make it impossible to base quantitative descriptions of borderline of deficiency on a hypothesis of normal distribution. The best evidence on this point is probably the data of Norsworthy with eleven tests on groups of 100 to 150 feeble-minded children in institutions and special classes and 250 to 900 normal children. She expressed the position of each child in terms of the deviation of the group of normal children of his age for each test. Pearson has presented her data graphically on the assumption that her defective group represented 0.3% of a general population of 50,000 children, and then fitted a normal distribution curve to her data with her normal group. The result makes it evident, especially for the intelligence tests, that the defective group would better be described as part of a skewed distribution. To less extent this is also true for the maturity and memory tests (_15_, p. 30). Norsworthy's own table of data show that 43 of the 74 feeble-minded taking the intelligence tests were over -5 times the probable error of their ages below the averages of the normal children, a criterion which she proposes as indicating ability outside of that included in the normal species. Moreover, 9 children score between -22 P. E. and -32 P. E. which is far beyond any conceivable extension of the normal curve. Her figure for the composite results of all her mental tests is also manifestly skewed toward deficiency although she hesitates to adopt this conclusion, and was content with showing that they grade off into the distribution of normal children.