Z Table Score Calculator allows you to quickly calculate z-scores. Background: In statistics, the z score (also called a z-value, standard score, or normal score) is the signed number of standard deviations by which an observation or data is above the mean. A positive z score indicates a datum above the mean, while a negative z score indicates a datum below the mean. It is a dimensionless quantity obtained by subtracting the population mean from an individual raw score and then dividing the difference by the population standard deviation. This conversion process is called standardizing or normalizing. The z-score is only defined if one knows the population parameters; if one only has a sample set, then the analogous computation with sample mean and sample standard deviation yields the Student's t-statistic.