A z-score is basically a standardized variable that has been rescaled to have a mean µ of 0 and a standard deviation σ of 1 (which ultimately provides a standard set of z-values - from the z-table - that can be used for easy calculations). It is calculated with the following formula: 𝑧 = (𝑋 − 𝜇) ⁄ 𝜎 (where 𝑋 is the data point, 𝜇 is the population mean, and 𝜎 is the population standard deviation). A z-table, also called standard normal table, is a table used to find the percentage of values below a given z-score in a standard normal distribution.Ī z-score, also known as standard score, indicates how many standard deviations away a data point is above (or below) the mean.Ī positive z-score implies that the data point is above the mean, while a negative z-score indicates that the data point falls below the mean.
