46 PSYCHOLOGICAL METHODS OP TESTING INTELLIGENCE 



The principle governing this distribution has, how- 

 ever, heuristic value even now in two ways: (1) 

 When we are obliged to divide a group of persons on 

 the basis of their mental ability into a good, a me- 

 dium, and a poor group, the convenient and common 

 division into three groups of equal size is certainly 

 less close to the actual gradations than the setting 

 off of a good and a poor quarter from the homoge- 

 neous middle half of medium ability. (2) A require- 

 ment, e. g., of a test or of a series of tests, may stand 

 as normal for a given group of individuals when ap- 

 proximately 75 per cent, of the members of the group 

 meet it in a satisfactory or more than satisfactory 

 manner. This idea has been used by Bobertag 7 for 

 the standardization of tests. 



(b) Different age-levels and nationalities. God- 

 dard has thought that the symmetry of the curve of 

 which we have just been speaking might be deemed 

 proof that the Binet-Simon arrangement of tests 

 represents in a way an ideal series, because it has af- 

 forded on empirical test a distribution that was 

 theoretically to have been expected. But this con- 

 clusion is unjustifiable. The curve of symmetry ap- 

 plies primarily only to all children taken collec- 

 tively, without regard to age; but the Binet-Simon 

 tests should really embrace normal standards for 

 children of every one of the series of ages and their 

 correctness would be demonstrated only provided 

 the symmetrical distribution were disclosed for 

 normal unselected children of each single year. But 

 this is by no means the case, and, as a matter of fact, 



T See Section 5a for details. 



