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In mathematics and statistics, **deviation** is a measure of difference for interval and ratio variables between the observed value and the mean. The sign of deviation, either positive or negative, indicates whether the observation is larger than or smaller than the mean. The magnitude of the value reports how different (in the relevant numerical scale) an observation is from the mean. One of the features of the mean is that the sum of the deviations across the entire set of all observations is always zero.

Average deviation is calculated using the absolute value of deviation - it is the sum of absolute values of the deviations.

Standard deviation is a more frequently used measure which uses squared deviations.

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