K. Pearson 
111 
gence of the median individual*, hence intellectual grades will be measured by a 
plus or minus number of mentaces above the median. The median individual 
must be looked upon as one whom, if a child, an experienced teacher would 
hesitate to class in the intelligent or slow intelligent groups, or who would bo 
considered a doubtful honours candidate at Cambridge. Clearly 100 mentaces is 
not far from the standard deviation of intellectual power in manf . 
The following is the suggested classification : 
(i) Genius\, more than + 300 mentaces. This corresponds to individuals of 
more than three times the standard deviation from the mean. 
(ii) Specially Able, +100 to +300 mentaces. This corresponds to quick 
intelligent children and first class University honours. 
(iii) Capable, +45 to + 100 mentaces. This corresponds to second class 
University honours. 
(iv) Fair Intelligence, + 0 to + 45 mentaces. This corresponds to third class 
University honours. Groups (iii) and (iv) cover the ground occupied by the school 
teacher's classification of "intelligent," and comprise about one-third of the total 
population. 
(v) Slow Intelligent, — 0 to — SO mentaces. 
(vi) Slow, — 80 to — 144 mentaces. 
(vii) Slow Dull, — 144 to — 208 mentaces. 
(viii) Very Dull, — 208 to — 300 mentaces. This group passes into the 
mentally defective. Groups (v) to (viii) occupy the range corresponding to pass 
men from the academic standpoint. 
(ix) Imbecile, less than — 300 mentaces. 
We may look upon the whole scheme from another .standpoint, corresponding 
more closely to Francis Gallon's decile arrangement, though unfortunately we can 
only a posteriori determine our class indices, and cannot make them suitable round 
numbers. 
Taking 1000 individuals and arranging them in intellectual order: The first, 
one man in a thousand, would be a genius, then follow 157 specially able men, next 
168 capable men, then 174 of fair intelligence ; this covers the 50 per cent, above 
the average. Below the average we have first 288 of the slow intelligent type, 
then 137 slow persons, next 56 slow dull, followed by 18 very dull including 
mentally defective individuals, and finishing with one imbecile ; thus completing 
the 50 per cent, with less than average intelligence. 
This scale is represented in the upper part of Fig. 1. 
* He can hardly have more than 350 to 400 mentaces, for at a negative position of - 350 to - 400 
on the scale we have passed through the very dull group into imbecility and complete absence of reason- 
ing power. The child whose low grade of intelligence occurs only 3 or 4 times in 100,000 cases, must 
be sought in the idiot asylum. 
t The standard deviation of school girls is 9G'8 mentaces, of school boys 90'3, and of graduate males 
92'9, or an average of 93-3 mentaces. The females thus appear more variable than the males in 
intelligence. 
% This is of course, purely arbitrary, the simple quantitative idea of "one man in a thousand." 
