THE METHOD OF AGE GRADATION 79 



feeble-mindedness being present, but a backwardness 

 of two, or of three years, respectively, for these ages, 

 certainly cannot coexist with normal intelligence. " 



TABLE XI 

 AVERAGE RETARDATION, IN YEARS, OF THE CHILDREN IN TABLE X 



Chronological Not Doubtful 



Age Feeble-minded Defect Morons Imbeciles 



8 0.65 1.3 1.9 2.3 



9 1.4 1.7 2.1 3.1 



10 2.0 2.0 2.6 3.8 



11 3.0 3.5 3.2 4.0 



12 2.0 3.0 3.3 4.7 



13 3.5 



That the size of the absolute difference for the 

 same degree of feeble-inindedness should increase 

 as age increases is psychologically easily intelligible, 

 for, since feeble-mindedness consists essentially in 

 a condition of development that is below the normal 

 condition, the rate of development will also be a 

 slower one, and thus every added year of age must 

 magnify the difference in question, at least as long 

 as there is present anything that could be called 

 mental development at all. With this in mind it is 

 but a step to the idea of measuring the backward- 

 ness by the relative difference, i. e., by the ratio be- 

 tween mental and chronological age, instead of by 

 the absolute difference. Bobertag had already con- 

 ceived a plan of this sort, while Kramer (54, p. 30) 

 hints at something of the sort, though very guard- 

 edly: "Whether perhaps there might be devised a 

 specific method of calculation for relating the dif- 

 ference in years to chronological age and which 

 would then give us an absolute measure for degree 

 of feeble-mindedness, seems to me a matter of 

 doubt." 



