A standard 'z-score' is related to:

A z score is a statistic used to represent as to how many standard detion intervals a value falls above or below the mean. It is also known as standard normal scores. The z score is most commonly used in the context of population and the same principles are used for identification of location within the sample. z scores can be more and less than the mean and thus considered as positive and negative z scores. It is calculated as: z = (X - M)/SD where X = One score in the example (Sample), sign of the z score depends as to whether the sample value is above or below the mean. M = Sample mean. SD = Standard Detion of the sample. Ref: Statistics for the Behavioral Sciences By Frederick J. Gravetter, Larry B. Wallnau, Page 153-154; Encyclopaedia of Mathematics : In 6 Banden By Michiel Hazewinkel, Volume 1, Pages 397; Statistics for Evidence-Based Practice and Evaluation By Allen Rubin, Pages 86-90.