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- [statistics] A statistical measure of the spread of values from their mean, expressed in standard deviation units, in which the z-score of the mean value is zero and the standard deviation is one. In a normal distribution, 68 percent of the values have a z-score of plus or minus 1, meaning they are within one standard deviation of the mean. Ninety-five percent of the values have a z-score of plus or minus 1.96, meaning they are within two standard deviations of the mean; 99 percent of the values have a z-score of plus or minus 2.58. Z-scores are a common scale on which different distributions, with different means and standard deviations, can be compared.