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A lower-case sigma, or a letter s, standing for the spread of a distribution.
Also known as: Sigma standard deviation, SD
Standard deviation measures how far values typically fall from the mean. Convention writes the population value as σ and the sample estimate as s — and the two are computed slightly differently, which is the reason for keeping separate letters.
Denotes the standard deviation: the square root of the average squared deviation from the mean, in the same units as the data.
Contexts: Mathematics
Written σ for a population and s for a sample. The sample formula divides by one fewer than the number of observations, which corrects a bias the population formula does not have.
The same Greek letter carries several unrelated meanings elsewhere in mathematics, including summation in its capital form and the sum-of-divisors function.
A lower-case Greek sigma — a closed loop with a short horizontal stroke leaving its top right — or a plain letter s.
Mean and standard deviation are quoted together as the summary of a distribution.
The same Greek letter. Lower-case sigma is a standard deviation in statistics; the capital is the summation sign in mathematics, and they are unrelated uses of one letterform.