Some common standard scores and their corresponding mean and standard deviation are: ACT scores (18 and 5), SAT scores (500 and 100), T-scores (50 and 10), and z-scores (0.0 and 1.0). A positive standard score indicates a datum above the mean, while a negative standard score indicates a datum below the mean. It is a dimensionless quantity obtained by subtracting the population mean from an individual raw score and then dividing the difference by the population standard deviation.