- standard deviation
- (sd)a mathematical unit used to describe the "spread" or dispersion of a set of data. Each item in the data set has a deviation from the mean (the ordinary average) of the data. The standard deviation is computed by taking the squares of these individual deviations, averaging these squares, and then taking the square root. If the data set conforms to a known distribution, such as the normal ("bell curve") distribution, then one can compute the percentage of the data which will fall within a certain distance of the mean, as measured in standard deviations. For example, if the data set conforms to the normal distribution, 68.3% of the data will fall within one standard deviation of the mean and only about 4.5% will fall outside 2 standard deviations from the mean.
Dictionary of units of measurement. 2015.
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