Sometimes also called Z-ratio or Z-value,
it is an example of what is called a 'Standard Score'. Z-scores are
typically associated with a normal distribution, and the interpretation is
based on a standard normal look up table. Z-scores are restricted to
distributions that approximate a normal distribution. Non-normal
distributions could be changed into Z-scores however. When this is done, a
standard normal table cannot be used, due to the fact that the shapes of
the Z-Score distributions in the table are designed for a normal
A conversion, it represents the distance in
sigma of a given sample from the population's mean,
sometimes referred to as X-Bar.
The formula for computing Z-Score is:
Where, X is the mean, Population Mean is
X-Bar, and sigma is standard
The resulting Z value is looked up in a
table to find a probability of obtaining a value equal to or less than X.