Whether it be speed in marking off all the A's in a printed
sheet of capitals, or in putting together the pieces of a puzzle, or in
giving a reaction to some certain stimulus, or in making associations
between ideas, or drawing figures, or memory for various things, or
giving the opposites of words, or discrimination of lifted weights, or
success in any one of hundreds of other mental tests, the conclusion is
the same. There are wide differences in the abilities of individuals, no
two being alike, either mentally or physically, at birth or any time
thereafter.
[Illustration: ORIGIN OF A NORMAL PROBABILITY CURVE
FIG. 10.--When deviations in all directions are equally
probable, as in the case of shots fired at a target by an expert
marksman, the "frequencies" will arrange themselves in the manner shown
by the bullets in compartments above. A line drawn along the tops of
these columns would be a "normal probability curve." Diagram by C. H.
Popenoe.]
Whenever a large enough number of individuals is tested, these
differences arrange themselves in the same general form. It is the form
assumed by the distribution of any differences that are governed
absolutely by chance.
Suppose an expert marksman shoots a thousand times at the center of a
certain picket in a picket fence, and that there is no wind or any
other source of constant error that would distort his aim.
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