σ sigma
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A lower case s is the symbol.
The symbol for standard deviation is pronounced as "sigma" in statistics. It is represented by the Greek letter σ. In mathematical and statistical contexts, σ is commonly used to denote the standard deviation of a population or a sample.
No. But they are related. If a sample of size n is taken, a standard deviation can be calculated. This is usually denoted as "s" however some textbooks will use the symbol, sigma. The standard deviation of a sample is usually used to estimate the standard deviation of the population. In this case, we use n-1 in the denomimator of the equation. The variance of the sample is the square of the sample's standard deviation. In many textbooks it is denoted as s2. In denoting the standard deviation and variance of populations, the symbols sigma and sigma2 should be used. One last note. We use standard deviations in describing uncertainty as it's easier to understand. If our measurements are in days, then the standard deviation will also be in days. The variance will be in units of days2.
Since this is regarding statistics I assume you mean lower case sigma (σ) which, in statistics, is the symbol used for standard deviation, and σ2 is known as the variance.
Sigma is a Greek symbol used for many variables in mathematics and science. However, within statistics, the variance is sigma squared and it is always positive. Sigma is used as the standard deviation of a population, and as calculated, it is always equal or greater than 0 (a positive number). However, in discussing errors, one can consider adding or subtracting sigma, ie the error in our experiment is +/- one sigma from the population.