Correlation and Association, continued.
Standard units are a way of putting different kinds of observations on the same scale.
The idea is to replace a datum by the number of standard deviations it is above the mean of the data. A datum that is above the mean by 2.5 times the SD is 2.5 in standard units. In fact, that is what it means for a set of data to be in standard units. Standard units are dimensionless. The difference between and the mean is which is SD s, so standard units.