The intelligence quotient, or IQ, was originally defined as the ratio of mental age to chronological age, multiplied by 100. A child who performed at the level of the average 10-year-old and who was herself 8 years old would have an IQ of 125.

Modern IQ tests no longer use the mental-age formula. Instead they compare an individual’s performance with the performance of a large, representative sample of the same age. The average is set at 100, and the standard deviation is typically 15. Scores are distributed in an approximately normal curve. Tests are standardized, and norms are periodically updated to account for population changes.

IQ scores predict academic achievement and, to a lesser extent, occupational success. They are less predictive of creativity, practical skill, or emotional adjustment. Scores can change with development, education, and intervention, especially in childhood. Controversy surrounds the interpretation of group differences and the potential for cultural bias in test content and administration.

An IQ score is a summary of performance on a particular set of tasks relative to a norm group. It is a useful predictor, not a complete measure of a person’s worth or potential.