<P> In statistics, the standard score is the signed number of standard deviations by which the value of an observation or data point is above the mean value of what is being observed or measured . Observed values above the mean have positive standard scores, while values below the mean have negative standard scores . The standard score 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 (however, "normalizing" can refer to many types of ratios; see normalization for more). </P> <P> Standard scores are also called z - values, z - scores, normal scores, and standardized variables . They are most frequently used to compare an observation to a standard normal deviate, though they can be defined without assumptions of normality . </P>

The descriptive statistic that indicates the typical distance of a score from the mean is the