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Z Scores
The z score of an observation xi,
taken from a population with mean m and
standard deviation s, is denoted by z
and is calculated as follows:
z = (xi - m) / s
The z score is a measure of the number of standard deviations
that an observation is above or below the mean. Since s is never
negative, a positive z score indicates that the observation is
above the mean, a negative z score that the observation is below
the mean. Note that z is a dimensionless value, and is therefore
a useful measure by which to compare data values from two
different populations (and thus possibly measured by different
units).
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