Habit Formation : Discrimination Method 237 



experiments it has become apparent that the average of the 

 results given by five individuals of the same sex, age, and 

 condition of health, when kept in the same environment and 

 subjected to the same experimental tests, is sufficiently con- 

 stant from group to group to warrant its use as an index 

 of modifiability for the race. This expression, index of modi- 

 fiability, is a convenient mode of designating the average 

 number of tests necessary for the establishment of a perfect 

 habit of white-black discrimination. Hereafter I shall use 

 it instead of a more lengthy descriptive phrase. 



As an indication of the degree of accuracy of measurements 

 of the rapidity of learning which are obtained by the use of 

 5 individuals I may offer the following figures. For one of 

 two directly comparable groups of 5 male dancers which were 

 chosen from 16 individuals which had been trained, the num- 

 ber of tests which resulted in a perfect habit of white-black 

 discrimination was 92 ; for the other group it was 96. These 

 indices for strictly comparable groups of 5 individuals each 

 differ from one another by less than 5 per cent. Similarly, 

 in the case of two groups of females, the indices of modifia- 

 bility were 94 and 104. These figures designate the number 

 of tests up to the point at which errors ceased for at least three 

 successive series (30 tests). 



The determination of the probable error of the index of 

 modifiability further aids us in judging of the reliability of 

 the measure of the rapidity of learning which is obtained by 

 averaging the results for 5 individuals. For a group of 5 

 males (Table 43, p. 243) the index was 72 3.5 ; and for 

 a group of 5 females of the same age as the males and strictly 

 comparable with respect to conditions of white-black training, 

 it was io42.9. A probable error of 3.5 indicates the 

 reliability of the first of these indices of modifiability; one 

 of 2.9, that of the second. 



