What an IQ score represents

An IQ score expresses how your test performance compares with a reference group of the same age, scaled so the average is 100 and about two-thirds of people fall between 85 and 115. It is a relative position, not an absolute amount of intelligence. A score of 120 means performance above most of the comparison group on those tasks, not "1.2 times" the ability of someone scoring 100.

A percentile says directly how many people in the comparison group scored lower. It also makes the shape of the distribution visible, and that shape is uneven. Going from 100 to 110 moves you from about the 50th to about the 75th percentile, overtaking roughly a quarter of the comparison group. Going from 130 to 140 moves you from about the 98th to about the 99.6th, overtaking under 2 percent. The same 10 points separate far more people in the middle than at the extremes, which is why percentiles communicate better than raw scores.

Test designers set the scale by administering the test to a large standardization sample and converting raw scores into standardized ones. This means an IQ score always depends on who the comparison group is and when the norms were collected. Because average raw performance shifted upward over the twentieth century, norms are periodically updated, and an old test can produce scores that look higher than a current one would.