A *z* score (or standardized value) is measured in units of the standard deviation. It tells you how many standard deviations the value *x* is above (to the right of) or below (to the left of) the mean, *μ*. Values of *x* that are larger than the mean have positive *z* scores, and values of *x* that are smaller than the mean have negative *z* scores. If *x* equals the mean, then *x* has a zero *z* score. It is important to note that the mean of the *z* scores is zero, and the standard deviation is one.

*z* scores help to find the outliers or unusual values from any data distribution. According to the range rule of thumb, outliers or unusual values have *z* scores less than -2 or greater than +2.

*This text is adapted from* *Openstax, Introductory Statistics, **6.1 The Standard Normal Distribution*

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