A Z score, or standard score, is the number of standard deviations an observation is away from the mean of the corresponding reference population. It is a measurement of the value of a single observation in relationship to the scores of a group of observations (population). Z scores transform units of analysis into a standardized form, allowing for comparison of variables measured in different units. When population parameters are known, Z scores are powerful for locating an individual observation in relation to all observations. When population parameters are not known, the Z distribution (normal distribution) changes slightly. As such, Z scores are a fundamental concept in statistical analysis. The transformation of units of analysis into standardized units (units of standard deviation) can be seen in ...