Standard deviation (s, the sample standard deviation, or σ, the population standard deviation) is another way to consider variation within our data around the mean.  It is calculated simply by taking the square root of the variance. Doing this computation means that the standard deviation is in the unit of the original variable, and is most useful for describing the spread of observations around the mean.

In short, the sample standard deviation is computed as:   s=√s² 

See variance link for how to calculate s².

In Excel, this is computed with the *STDEV.S* function.

Excel

=STDEV.S(cell:cell)

So, for the crabs we have observed that the sample standard deviation is 1.21 mm and the sample variance is 1.46 mm squared.