THE METHOD OF AGE GRADATION 39 



damental psychological fact (already mentioned 

 above) that, on account of the purely formal char- 

 acter of intelligence and the lack of complete cor- 

 relation among its constituent capacities, there never 

 is a real phenomenological equivalence between the 

 intelligence of two persons: what we do have is 

 rather a teleological equivalence when measured in 

 terms of the single function of all intelligence, 

 namely, adaptation to new requirements. And for 

 this equivalence of two intelligences mental age fur- 

 nishes an approximate measure, despite the fact that 

 their equivalence does not mean their identity. 



The area of irregularity yet further affects the computation of 

 mental age and in a way to which sufficient attention has not al- 

 ways been given. In order to equalize possible omissions in the 

 lower test-levels, one must always have at one's disposal tests in 

 higher levels. Now the original Binet-Simon series comprised 

 tests up to 13 years only : it follows that mental age 12 or 13 can- 

 not be correctly computed, for tests from yet higher levels might 

 perhaps have raised the total performance to a higher value. In 

 using the 1908 Binet series, accordingly, computations ought to be 

 carried up to mental age eleven only. 



The area of irregularity, again, affords another 

 value in addition to mental age, viz.: the range of 

 irregularity (Streuungsbreite). A child whose suc- 

 cesses and failures are strewn irregularly over test- 

 levels from 6 to 10 years has the same mental age, to 

 be sure, but a very different range of irregularity, 

 when compared with another whose mixture of suc- 

 cesses and failures lies in the 7th to the 9th years 

 only. Bobertag, who first gave attention to the im- 

 portance of differences in ranges of irregularity, 

 has devised a way of computing this factor ; I have 

 myself suggested another way, but neither has been 

 published as yet. 



