On account of the great attention that has been paid to individual differences in recent years, on account of their importance for diagnosis, for determining the causes of deficiency, and for planning for the training of deficients, we have come almost to the point where we forget the significance of the average as the most common condition with which we have to deal. The lack of complete correlation between abilities of an individual does not make us hesitate to use the concept of his average ability; it should not make us neglect or misunderstand the significance of the position of an individual testing low down on the scale. For the problem of social care the borderline position on a scale is immensely more important than higher ability. It seems advisable, therefore, to define this borderline ability with some suitable allowance for variability in mental processes. It is far safer to judge an individual's chance of survival by his average or general tested ability than by the little knowledge that is as yet available regarding special abilities. ----- Footnote 4: AUGUSTA F. BRONNER. _The Psychology of Special Abilities and Disabilities._ Boston, 1917, pp. vii, 269. CHAPTER IV. WHAT PERCENTAGE IS FEEBLE-MINDED A. KINDS OF SOCIAL CARE CONTEMPLATED At first it seems like a hopeless task to try to bring harmony out of the confused estimates of the proportion of the feeble-minded in modern society. Authoritative estimates by commissions or by recognized experts range from less than 0.2% to 5.0% that is, from 2 to 50 per thousand. Further study of these estimates shows that they reflect not so much a difference in expert opinion about the same problem as differences in the problems which were considered in making the estimates. As soon as we compare only those estimates that have been made to answer the question, what percentage of low grade minds should be provided with a certain form of social care? it is rather surprising how much less the discrepancy becomes. An analysis of important estimates will therefore be undertaken in order to try to discover some of the sources of disagreement.