It is the lower case Greek letter sigma. I cannot show it here because this browser converts it to the Roman letter s, but the Greek one looks like an o with a tilde attached to its top.
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Sigma
σ (sigma)
s is the sample standard deviation. it is computed by taking the square root of: sum(x-mean)2/n-1
N is neither the sample or population mean. The letter N represents the population size while the small case letter n represents sample size. The symbol of sample mean is x̄ ,while the symbol for population mean is µ.
the small greek letter sigma squared.