THE METHOD OF AGE GRADATION 41 



But let us return to mental age. The full signifi- 

 cance of this final value is disclosed only when we 

 consider it in relation to other circumstances. It can 

 evidently be related to other quantitative scales, like 

 chronological age, school grade and school standing, 

 or we can find out how it varies with certain qualita- 

 tive conditions, like social level, type of school, na- 

 tionality and the like. 



Doubtless most significant is the relation of mental 

 age to the actual chronological age of the subject, for, 

 as already said, a certain mental level goes normally 

 with a certain age, so that the relation of mental to 

 chronological age indicates the amount of discrep- 

 ancy between the intelligence present and that re- 

 quired (in the sense of a norm to be expected), and 

 in this way affords an expression for the degree of 

 the child's intellectual endowment. 



Up to now this discrepancy has always been com- 

 puted in the simple form of the difference between 

 the two ages, which, when negative gave the absolute 

 mental retardation, when positive the absolute mental 

 advance of the child in terms of years. Thus, if 

 mental retardation = - - 2, the child's mental de- 

 velopment is two years behind the normal level of 

 his age. 



It is perfectly clear how valuable the measurement 

 of mental retardation is, particularly in the investi- 

 gation of abnormal children. It has, however, been 

 shown recently that the simple computation of the 

 absolute difference between the two ages is not en- 

 tirely adequate for this purpose, because this differ- 

 ence does not mean the same thing at different ages 

 (compare what is said in Section 4a, pp. 70 ff.) . Only 



