Z-scores

The standardised form of a variable X is found by subtracting its mean then dividing by its standard deviation,

standardised value,   

The resulting values are called z-scores and are the same, whatever the units in which X was originally recorded.

Properties of z-scores

A standardised variable always has zero mean and standard deviation one.

From the 70-95-100 rule-of-thumb,

An individual's z-score tells you how many standard deviations it is above the mean. From its value, you can tell whether the value is very high (say over +2) or low (say under -2) in relation to the other values of the variable.