StdDev(x1,x2 [,..,x20]: Calculates the SD of the parameters.
RelStdDev(x1,x2 [,..,x20]: Calculates the RSD of the parameters.

Stdandard Deviation assumes that its arguments are a sample of the population. The standard deviation is calculated using the 'unbiased' or 'n-1' method. N/A values are skipped

Relative Standard Deviation
Relative SD is calculated as follows: RSD = 100 * (SD/Abs(mean))