Give an intelligence test to thousands of people, and the scores spread into a familiar shape. Most bunch up near the middle. Few land at the far ends. This shape is a normal distribution, or bell curve. Slide a score to see where it falls. Then flip to the dartboard to learn the two questions we ask of any test. Is it reliable? Is it valid?
IQ on the normal curve
IQ score100
About 68% of scores fall from 85 to 115 · 95% from 70 to 130 · 99.7% from 55 to 145
Each dartboard is one test. The bullseye is the true thing you want to measure.