Z Score
Sometimes also called Zratio or Zvalue,
it is an example of what is called a 'Standard Score'. Zscores are
typically associated with a normal distribution, and the interpretation is
based on a standard normal look up table. Zscores are restricted to
distributions that approximate a normal distribution. Nonnormal
distributions could be changed into Zscores however. When this is done, a
standard normal table cannot be used, due to the fact that the shapes of
the ZScore distributions in the table are designed for a normal
distribution.
A conversion, it represents the distance in
sigma of a given sample from the population's mean,
sometimes referred to as XBar.
The formula for computing ZScore is:


(X 
Population Mean) 
Z 
= 
 


Sigma 
Where, X is the mean, Population Mean is
XBar, and sigma is standard
deviation.
The resulting Z value is looked up in a
table to find a probability of obtaining a value equal to or less than X.