PSYCHOLOGY: R. M. YERKES 
115 
various measurements in question are distributed among the most im- 
portant intellectual functions (affectivity being omitted), so that the 
several aspects — memory, imagination, perception, association, sugges- 
tion, judgment, ideation — may be measured. 
The value of the point scale depends upon reliable norms. Two 
boys, from poor surroundings, each of the age seven years, six months, 
subjected to examination, obtain, for example, the one a credit of twenty- 
two points and the other a credit of fifty-nine points. By reference to 
our norms, we discover that boys of the age in question born to EngHsh- 
speaking parents of poor or medium circumstances, should obtain a 
score of 36 — while those born to parents of good or superior circum- 
stances should obtain 45. Since these norms, as is evident, take account 
of sex, age, language, and social status, it is clear that the first indi- 
vidual is nearly 40 per cent below our reasonable expectation in intel- 
lectual ability, whereas the second individual is instead about 37 per 
cent above what we should expect. 
It thus is clear that instead of stating that an individual is a certain 
number of years and months above or below age, as in the Binet method, 
we can definitely state the relation of a given individual examined to 
a more or less inclusive group in which he belongs. This value we 
call the coefficient of mental abiHty. It is obtained by dividing the 
individual's score by the average or norm for his group. We may also, 
if it is desirable, state the exact frequency of occurrence in the group 
of the degree of intelligence indicated, as for example, one in one hun- 
dred, with respect to inferiority or superiority, as the case may be. 
A single measure chosen from our scale must serve to indicate its 
chief features. We choose the test for the span of auditory memory. 
The subject is required to repeat, after the experimenter, groups of 
digits ranging in number from three to seven. These are presented by 
the experimenter orally, clearly and distinctly, at the rate of two per 
second. If the subject fails to repeat correctly the first group of a given 
number, he is given a second trial with another group containing the 
same number. If he succeeds with that, the experimenter passes on 
to a group containing one additional digit, and so on. One point credit 
is allowed for each of the five groups correctly reproduced. If a subject 
fails in both trials for a given number of digits, the observation is ter- 
minated. The test for memory-span is: 
