In more detail
A mark of 85 with mean 70 and SD 10 has z = 1.5, about the 93rd percentile of a normal distribution. Z-scores let you compare values from different scales.
Glossary
Math & Statistics
z-score
A z-score says how many standard deviations a value lies above or below the mean: `z = (x − mean) / SD`.
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See also
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Glossary
Standard deviation
Standard deviation measures how far values typically stray from their mean; it is the square root of the variance.
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Glossary
Normal distribution
The normal distribution is the symmetric bell-shaped curve that describes many natural measurements.