27-4 What is an intelligence test, and what is the difference between achievement and aptitude tests?

An intelligence test assesses people’s mental abilities and compares them with others, using numerical scores. Psychologists classify such tests as either aptitude tests, intended to predict your ability to learn a new skill, or achievement tests, intended to reflect what you have already learned. How do we design such tests, and what makes them credible? Consider why psychologists created tests of mental abilities and how they have used them.

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27-5 When and why were intelligence tests created, and how do today’s tests differ from early intelligence tests?

Barely a century ago, psychologists began designing tests to assess people’s abilities. Some measured aptitude (ability to learn). Others assessed achievement (what people have already learned).

ALFRED BINET: PREDICTING SCHOOL ACHIEVEMENT Modern intelligence testing traces its birth to early-twentieth-century France, where a new law required all children to attend school. French officials knew that some children, including many newcomers to Paris, would struggle and need special classes. But how could the schools make fair judgments about children’s learning potential? Teachers might assess children who had little prior education as slow learners. Or they might sort children into classes on the basis of their social backgrounds. To minimize such bias, France’s minister of public education gave Alfred Binet (1857–1911) the task of solving this problem.

In 1905, Binet and his student, Théodore Simon, first presented their work under the archaic title, “New Methods for Diagnosing the Idiot, the Imbecile, and the Moron” (Nicolas & Levine, 2012). They began by assuming that all children follow the same course of intellectual development, but that some develop more rapidly. A “dull” child should score much like a typical younger child, and a “bright” child like a typical older child. Binet and Simon now had a clear goal: They would measure each child’s mental age, the level of performance typically associated with a certain chronological age. The average 8-year-old, for example, has a mental age of 8. An 8-year-old with a below-average mental age (perhaps performing at the level of a typical 6-year-old) would struggle with schoolwork considered normal for 8-year-olds.

Binet and Simon tested a variety of reasoning and problem-solving questions on Binet’s two daughters, and then on “bright” and “backward” Parisian schoolchildren. Items that the successful students more often answered correctly could then be used to predict how well other French children would handle their schoolwork. Binet hoped his test would be used to improve children’s education, but he also feared it would be used to label children and limit their opportunities (Gould, 1981).

LEWIS TERMAN: THE INNATE IQ Binet’s fears were realized soon after his death in 1911, when others adapted his tests for use as a numerical measure of inherited intelligence. Stanford University professor Lewis Terman (1877-1956) found that the Paris-developed questions and age norms worked poorly with California schoolchildren. He adapted some items, added others, and established new standards for various ages. He also extended the upper end of the test’s range from teenagers to “superior adults” and gave his revision the name it retains today—the Stanford-Binet. Terman assumed that certain ethnic groups were naturally more intelligent, and he supported the controversial eugenics movement, which aimed to protect and improve human genetic quality through selective sterilization and breeding.

From such tests, German psychologist William Stern derived the famous intelligence quotient, or IQ. The IQ was simply a person’s mental age divided by chronological age and multiplied by 100 to get rid of the decimal point. Thus, an average child, whose mental age (8) and chronological age (8) are the same, has an IQ of 100. But an 8-year-old who answers questions at the level of a typical 10-year-old has an IQ of 125:

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The original IQ formula worked fairly well for children but not for adults. (Should a 40-year-old who does as well on the test as an average 20-year-old be assigned an IQ of only 50?) Most current intelligence tests, including the Stanford-Binet, no longer compute an IQ in this manner (though the term IQ still lingers in everyday vocabulary as shorthand for “intelligence test score”). Instead, they represent the test-taker’s performance relative to the average performance of others the same age. This average performance is arbitrarily assigned a score of 100, and about two-thirds of all test-takers fall between 85 and 115.

DAVID WECHSLER: SEPARATE SCORES FOR SEPARATE SKILLS Psychologist David Wechsler created what is now the most widely used individual intelligence test, the Wechsler Adult Intelligence Scale (WAIS). There is a version for school-age children (the Wechsler Intelligence Scale for Children [WISC]), and another for preschool children (Evers et al., 2012). The WAIS (2008) edition consists of 15 subtests, including:

The WAIS yields both an overall intelligence score and individual scores for verbal comprehension, perceptual organization, working memory, and processing speed. Striking differences among these individual scores can provide clues to cognitive strengths or weaknesses. For example, a low verbal comprehension score combined with high scores on other subtests could indicate a reading or language disability. Other comparisons can help a therapist establish a rehabilitation plan for a stroke patient. In such ways, these tests help realize Binet’s aim: to identify opportunities for improvement and strengths that teachers and others can build upon.

27-6 What is a normal curve, and what does it mean to say that a test has been standardized and is reliable and valid?

To be widely accepted, a psychological test must be standardized, reliable, and valid. The Stanford-Binet and Wechsler tests meet these requirements.

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WAS THE TEST STANDARDIZED? The number of questions you answer correctly on an intelligence test would reveal almost nothing. To know how well you performed, you would need some basis for comparison. That’s why test-makers give new tests to a representative sample of people. The scores from this pretested group become the basis for future comparisons. If you later take the test following the same procedures, your score will be meaningful when compared with others. This process is called standardization.

If we construct a graph of test-takers’ scores, the scores typically form a bell-shaped pattern called the normal curve. No matter what attributes we measure—height, weight, or mental aptitude—people’s scores tend to form a bell curve. The highest point is the midpoint, or the average score. On an intelligence test, we give this average score a value of 100 (FIGURE 27.3). Moving out from the average, toward either extreme, we find fewer and fewer people. For the Stanford-Binet and Wechsler tests, a person’s score indicates whether that person’s performance fell above or below the average. A performance higher than all but 2 percent of all scores earns an intelligence score of 130. A performance lower than 98 percent of all scores earns an intelligence score of 70.

IS THE TEST RELIABLE? Knowing your score in comparison to the standardization group still won’t tell you much unless the test has reliability. A reliable test gives consistent scores, no matter who takes the test or when they take it. To check a test’s reliability, researchers test people many times. They may retest people using the same test, or they may split the test in half and see whether odd-question scores and even-question scores agree. If the two sets of scores generally agree, or correlate, the test is reliable. The higher the correlation, the higher the test’s reliability. The tests we have considered so far—the Stanford-Binet, the WAIS, and the WISC—are, after early childhood, very reliable (about +.9). When retested, people’s scores generally match their first score closely.

IS THE TEST VALID? High reliability does not ensure a test’s validity—the extent to which the test actually measures or predicts what it promises. Imagine using a miscalibrated tape measure to measure people’s heights. Your results would be very reliable. No matter how many times you measured, people’s heights would be the same. But your results would not be valid, because you would not be giving the information you promised: real height.

Tests that tap the pertinent behavior, or criterion, have content validity. The road test for a driver’s license has content validity because it samples the tasks a driver routinely faces. Course exams have content validity if they assess your mastery of course material. But we expect intelligence tests to have predictive validity: They should predict future performance, and to some extent they do.

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The predictive power of aptitude tests is fairly strong in the early school years, but later it weakens. Past grades, which reflect both aptitude and motivation, are better predictors of future achievements.