The concept of average intelligence is sometimes difficult to appreciate because the two words, average and intelligence, are sometimes not defined.
To psychologists and counselors who administer tests of intelligence, a person who scores at the 50th Percentile has average intelligence as defined by the number of correct answers to test tasks compared to others in their age group.
Many tests set the middle score at 100 thus, 100 = average intelligence on many tests.
All test scores vary from time to time so, a person may earn more or less points on another day. This fluctuation is estimated and can range for example by plus or minus 3-5 IQ points depending on the test and age group.
If you retake the test in a month or so, you may score better because of the “practice effect”—you’ve seen the items recently so you will probably do better.
There is an average range so examiners will not focus on the obtained score but consider a broader range. For example, some may consider 90 to 110 as average. Some use a statistic called the standard deviation, which is often 15 points on an IQ test. If a clinician uses a Standard Deviation of 15 points then the average range of intelligence scores = 85 to 115 (that is plus or minus 15 points from 100). Statistically, about 68% of people earn scores in this broad average range thus, most people in a given age group and the same population, will have an IQ score or scores in this broad average range.
By this definition, people who are above average intelligence earn scores above 115 on tests. In the US, schools often considered scores at 130 or higher as gifted but other tests and reports are considered. Also, people who scored below 85 were considered below average intelligence. Depending on their other abilities, they may need assistance with school work or work tasks. People with high and low scores are different so broad statements can be misleading.
There are different theories of intelligence and tests have been constructed based on a few of the theories. Clinicians should be able to tell you basic facts about the test you or your child/loved one took. For the most part, the best tests ask examinees to answer a variety of questions and solve different types of problems. Thus, the best tests sample a variety of problem-solving tasks and average the scores for the different types of tasks.
For example, the ability to define words is one common measure of verbal intelligence. Through many years, examiners have found what people know in different age groups.
An example of performance intelligence is solving puzzles using blocks with different designs, which can be arranged to match pictures on a card. This ability increases considerably from preschool to adulthood.
There are other types of intelligence like emotional intelligence and social intelligence. Clinicians have developed tests to measure these skills too.
In a sense, intelligence is what is measured by intelligence tests—that’s circular—but it does give people a sense of what people know how to do compared to their age peers.
In addition, when abilities decline due to disease or head injury, knowing what is average for a person of a given age can be helpful in understanding the loss and marking recovery or further decline.
As a matter of context, clinicians usually administer other tests and conduct an interview to avoid interpreting test scores out of context.
Average intelligence is therefore, a middle range of abilities compared to other people of the same age who have taken the same test.
Learn more about test and other statistics in
Applied Statistics for Counselors