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 Description  
 z-score:  A measure describing how close members of a data set are from the mean of the data set.
The z-score in a data set is the distance between a data point and the mean of the data set measured in standad deviations. It is also called "standard score". The z-score of a score indicates how far and in what direction that score deviates from its distribution's mean, expressed in units of its distribution's standard deviation. It is calculated by

or

Finding the z-score is especially useful when seeking to compare the relative standing of scores or values from distribution with different means and/or different standard deviations.