Statistical measure of the variance of a distribution of values
expr - data series formula
count - lookback period
StdDev is calculated in the same way that Excel calculates the STDEV.P function, which is as if the set of count values is the entire population.
The specific formula used is shown below, in the "sdev" item:
This also illustrates how these statistical functions could be calculated in the Data Section of a script, though since they're provided built-in, there's no reason to do so.
See also Skewness and Kurtosis.