Intelligence tests (IQ tests) are in the news lately as people banter about terms from many decades ago. IQ tests are widely used because they measure the ability of people to solve various problems, predict academic achievement, and help with job placement in some settings. The tests also help neuropsychologists assess functioning in people with impairments due to head injuries and brain diseases.
During part of my childhood, I passed a facility where American IQ testing began. I saw people on swings and on the grounds of the Vineland Training school in Vineland NJ. It turns out that a little over 100 years ago, American psychologist, Henry Goddard, brought a test by French scientist, Alfred Binet, to the New Jersey Training School for Feeble-Minded Girls and Boys in Vineland, NJ. The test was modified and widely used in the U.S.
What tests are used today?
Today, a number of tests are available in the US and elsewhere. Popular American tests are the Wechsler Intelligence Scales and the Stanford-Binet Scale. The tests are regularly updated with new materials and tasks appropriate to people of different ages. Several other tests are also available such as the Kaufman Assessment Battery, Woodcock-Johnson Tests of Cognitive Ability, and the Differential Ability Scales.
A full scale test can last over an hour, so it is not surprising that a number of shorter tests are available. The shorter tests are considered "screening tests" because they include fewer subtests (or sets of tasks), to measure problem-solving skills. It is common to use a test of verbal ability such as vocabulary and a test of "nonverbal" ability such as tasks that require solving visually presented tasks.
Intelligence tests yield a variety of scores that recognize people have different abilities. This fits with common sense as we observe people with different abilities--strong verbal skills, incredible abilities to design complex structures, create various artistic works, and so forth. Still, many people want to know their IQ-- a short hand way of identifying an overall general ability. The overall score is controversial but remains in use.
In years past, the IQ (intelligence quotient) was measured as a ratio of chronological age to mental age. Mental age referred to a person's score on test tasks compared to others of the same age. As I posted previously, there are problems with age scores. Today, the scores on tests of intelligence compare people of similar ages to their age peers. For traditional reasons, the average IQ (or standard score) has an arithmetic average (M, mean) of 100 and a standard deviation (SD) of 15 points (read more about a few statistics).
It turns out that despite different test tasks and scales, people earn similar scores. An IQ score or standard score on one test is likely within a few points of the same score on a different test. As people age, the scores are more reliable-- that is stable. So, if an adult earns a score of 110 today, she would likely have a score within a few points of 110 in 2-years--unless something happened.
The stability of the scores make the tests useful when considering the effects of brain damage or disease. Of course, neuropsychologists use other tests as well (e.g., tests of memory, visual-spatial skills).
What is average intelligence?
That can be a trick question unless you clarify what you mean by intelligence. On tests that report standard scores, the average score is 100. Using the common standard deviation of 15 points, about 68% of our age peers will score beetween 85 and 115. Close to 95% of people the same age will have scores in the range of 70 to 130. As you can see, only a small percentage of people score above 130 or below 70.
The test scores compare people to others of the same age. The skill levels develop rapidly in young children. Several months can make a difference in average scores. In adults, scores vary in how they change for people in large age brackets. Some abilities decline more rapidly than others. For example, young adults tend to be faster than older adults when solving tasks requiring eye-hand coordination.
What are some problems with IQ tests?
Test scores do not capture the range of abilities of people who are differently abled. For example, those with severe visual impairments cannot see visual test tasks. And those with severe hearing impairments may not respond well to spoken instructions or auditory tasks. Clearly, it would be wrong to assume something about a person's intelligence using tests that are not designed for people who have limited vision, hearing, or some other similar condition.
Some clinicians fail to document vision, hearing, or other limitations. For example, many people show up for testing and leave their eyewear at home. A child may forget his glasses or hearing aids.
People with temporary limitations cannot take tests as well as they could at other times. If you cannot use your dominant hand due to an injury, you will have difficulty on tests that require using your hands.
People taking medication can respond differently when taking medicine that either helps or interferes with attention and concentration. Of course, illegal drugs can also affect the brain processes needed to remember instructions and solve problems.
People who are not fluent in the language of the test may have a difficult time depending on their language skills.
So called "nonverbal tests" measure different abilities than tests that include language so mistakes can be made when making judgments about general intelligence or ability.
Clinicians make mistakes in recording information, scoring, or writing reports.
Tests are not perfect measuring instruments. Even when administered to people under the best of circumstances, there is measurement error. Measurement error is usually more variable for children than for adults. Measurement error refers to a variation in scores from one administration to another.
What about labels?
I suppose we will have a hard time escaping labels. The words used for people getting high scores or low scores have changed over the years--too many to cover in this post. Insulting words about a person's intelligence were terms used many decades ago. Today, clinicians and organizations like schools use a variety of terms focused on helping high scoring students learn in more challenging environments. And students who score very low on several tests, are elligible for services designed to help them maximize their potential. Insurance programs use cutoff scores and other criteria when awarding benefits to people with severely impaired abilities.
Who administers IQ tests?
A variety of professionals are qualified to administer, score, and interpret IQ tests. They are most commonly used by School Psychologists in schools and private practices. But other psychologists who specialize in neuropsychology also use IQ tests as part of their assessment. Many school counselors also have the necessary skills. In some cases, a psychological technician will administer the tests but the interpretation is left to the clinian holding an advanced degree along with the appropriate license or certification.
Applied statistics: Concepts for counselors.
You can also read more about the assessment of"thinking" in Creating Surveys.
Books by Geoffrey W. Sutton