James McKeen Cattell, an American psychologist, soon built upon Galton's physiological method of measuring intelligence. In 1890, he published a set of "mental tests," a catchy term he coined. Cattell suggested 10 mental tests for use with the general public.
• Dynamometer pressure. This test measured the strength of a person's hand grip. Cattell explained that he included this test because "it is impossible to separate bodily from mental energy."
• Rate of hand movement. This test measured how quickly a person could move his or her hand across 50 centimeters.
• Two-point threshold. A researcher touched a pair of rubber-tipped compass points to the back of a person's hand. When the tips were very close together, the subject felt them as a single point. The researcher attempted to find the smallest distance at which the tips were felt by the subject as two separate points.
• Pressure-causing pain. An instrument was pressed against a person's forehead with increasing force. The aim was to find the amount of pressure needed to cause signs of pain.
• Weight differentiation. This test required a person to put a set of identical-looking boxes in order by weight. The boxes, which differed in weight by 1 gram, ranged from 100 to 110 grams.
• Reaction time for sound. This test measured the very brief period that elapsed between the time when a sound was made and the time when a person's muscles started reacting to it.
• Time for naming colors. A set of red, yellow, green, and blue patches, arranged in random order, was shown to a person. The aim was to measure how long it took the person to name the colors.
• Bisection of line. A 50-centimeter strip of wood with a sliding line attached was used. The person was asked to place the line as close as possible to the exact middle of the strip.
• Judgment of time. In this test, the examiner first tapped out a 10-second interval. The examiner then tapped on the table and asked the person to signal when another 10 seconds had passed.
• Number of repeated letters. This test measured how well a person could repeat back lists of random consonants.
A flurry of this kind of mental testing followed in the 1890s. When powerful new statistical methods came into use, however, it soon became clear that the tests were sorely lacking. Earlier, Galton had developed the concept of a correlation, the degree and direction of association between two things. Karl Pearson perfected the method of computing a correlation coefficient, an index of the strength of the relationship between two things when certain conditions are met. This statistic became known as the Pearson r. Now, researchers had a more sophisticated way to analyze test results.
In 1901, Clark Wissler, one of Cattell's own graduate students, dealt a death blow to this type of mental testing. Using the new statistical methods, he studied the scores of college students who had taken Cattell's tests. Wissler found virtually no correlation among the tests. In other words, a student who did well on one of the tests was not especially likely to do well on any of the other tests. Even worse, scores on the tests also did not correlate with college grades. This meant Cattell's tests and college grades were measuring different things. Since college grades were thought to reflect intelligence, it seemed Cattell's tests must be measuring something else.
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