How do test norms and percentiles help interpret intelligence-test scores?

Test norms and percentiles are crucial for interpreting intelligence test scores by providing a reference point for comparison. A raw score, such as the number of correct answers, has limited meaning on its own. Test norms allow an individual's score to be understood in relation to the performance of a specific group, often referred to as the normative group.1

Percentiles, also known as centiles, indicate the percentage of individuals in the normative group who scored below a particular raw score. For example, if a person scores at the 70th percentile, it means their score is higher than approximately 70 percent of the scores in the reference group. This allows for a clear understanding of an individual's relative standing, indicating whether they performed better or worse than the average.213

Another type of norm system uses standard scores, which are based on how far a raw score deviates from the average score (the mean). Historically, intelligence quotients (IQs) were calculated as a ratio of mental age to chronological age, but modern tests primarily use deviation IQs, which are standard scores with a set mean and standard deviation for each age level.1