Let x denote the values of the variable in question.
Suppose there are n observations.
Let Sx = the sum of all the values.
then the mean of x, Mx = Sx/n
Let Sxx = the sum of all the squares of the values.
The Vx (= the variance of x) is Sxx - (Mx)^2
and sigma(x) = sqrt(Vx).
Therefore one sigma deviation, relative to the mean,
= Mx - sigma(x), Mx + sigma(x).
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Sigma
σ (sigma)
Yes, the variance of a data set is the square of the standard deviation (sigma) of the set. This means that the variance is always a positive number, even though the data might have a negative sigma value.
The letter s, or by sd. The Greek lower case sigma is also used.
the standard deviation of the population(sigma)/square root of sampling mean(n)