A variable (a set of data) is said to be in standard units if its mean is zero and its standard deviation is one. You transform a set of data into standard units by subtracting the mean from each element of the list, and dividing the results by the standard deviation. A random variable is said to be in standard units if its expected value is zero and its standard error is one. You transform a random variable to standard units by subtracting its expected value then dividing by its standard error.