In a later paper he says that those children “from 4 to 4.5 years and beyond of mental defect could not be matched at all from 27,000 children,” on the assumption of a normal distribution fitted to the normal Stockholm school children (164, p. 51). He says further:
“It is a matter of purely practical convenience where the division—if there must be an arbitrary one—between the normal and defective child is placed; we suggest that it be placed at either 3 or 4 years of mental defect. But as mental defect increases with the age of the mentally defective the division will be really a function of the child's age” (167, p. 37).
Since he finds the children in the special classes fall further behind the normal children on the average 4 months each year of life, this means that 3 years retardation at 7 years of age would be equivalent to 4 years at 10.
In spite of uncertainty introduced by the use of quotients, the general tendency in interpretation of results with Binet scales has thus been to make a distinction in the amount of retardation signifying deficiency among younger and older children and to require four years retardation, at least for the older ages. Our criterion for the borderline of three years retardation for children under 10 years and four years for 10 years and over, with an extra year to be quite sure that the deficiency is sufficient to justify isolation, seems to be in line with the best practise at present among those who have had much experience with the Binet scale. Fortunately, little harm has been done to the individuals themselves by this uncertainty in the interpretation of the scores with the scale, since only questionable cases have been affected. These have generally been diagnosed, before disposing of the child, by some expert who understands the sources of error in mental tests. On the other hand, shifting the limit of allowable retardation by one year makes a great difference in the estimation of the frequency of feeble-mindedness in particular groups, as will be shown in our discussion of deficient delinquents.
[11]. Throughout this study I shall use the literal translation of the German term “lebensalter,” life-age, instead of the awkward “chronological age.”
[12].
| S. E. = | √ | p. q. |
| n |
[13]. Tests XI were recorded as XII in the 1911 series.