Z-scores
The standardised form of a variable X is found by subtracting its mean then dividing by its standard deviation,
standardised value, | ![]() |
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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.