Z-score

A standardized score expressing how many standard deviations a raw score is from the mean.

A Z-score expresses a raw score in terms of standard deviations from the mean. A Z-score of 0 is exactly average; +1 is one standard deviation above the mean; -2 is two standard deviations below.

Z-scores are the foundation for most other standardized scores (T-scores, IQ scores, etc.) and let you compare scores across measures that use different raw-score scales. The drawback for general audiences is that Z-scores include negative values and decimals — which is why clinical assessments typically report transformed versions like T-scores instead.

T-score — A standardized score with a mean of 50 and a standard deviation of 10, commonly used in psychological testing.
Normal distribution — A symmetric, bell-shaped probability distribution that describes how many psychological traits are spread across a population.
Norms — Reference distributions of scores from a defined population, used to interpret an individual's score in context.
Percentile rank — The percentage of scores in a reference group that fall at or below a given score.

Z-score

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