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Z Score

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 distribution.

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:

(X – Population Mean)
Z = —————————-
Sigma

Where, X is the mean, Population Mean is X-Bar, 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.

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Last Updated: Saturday, 10-Jun-06 15:50:57 PDT

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