Z-score (standard score)
How many standard deviations a value lies above or below the mean.
A z-score expresses how far a single value is from the mean of its dataset, measured in standard deviations: z = (x − mean) ÷ standard deviation. A z-score of 0 is exactly average, +2 is two standard deviations above the mean, and −1.5 is one and a half below. Z-scores let you compare values from different distributions on a common scale and underpin probability lookups in a normal distribution.
Example: A test score of 80 where the mean is 70 and SD is 5 has z = (80 − 70) ÷ 5 = 2.