STDEVX.S Function (DAX)
Returns the standard deviation of a sample population.

STDEVX.S evaluates expression for each row of table and returns the standard deviation of expression assuming that table refers to a sample of the population. If table represents the entire population, then compute the standard deviation by using STDEVX.P.

STDEVX.S uses the following formula:
√[∑(x  x̃)²/(n1)]
where x̃ is the average value of x for the entire population
and n is the population size

Blank rows are filtered out from columnName and not considered in the calculations.

An error is returned if columnName contains less than 2 nonblank rows.
This DAX function may return different results when used in a model that is deployed and then queried in DirectQuery mode. For more information about semantic differences in DirectQuery mode, see http://go.microsoft.com/fwlink/?LinkId=219171.
The following example shows the formula for a calculated column that estimates the standard deviation of the unit price per product for a sample population, when the formula is used in the Product table.
=STDEVX.S(RELATEDTABLE(InternetSales_USD), InternetSales_USD[UnitPrice_USD] – (InternetSales_USD[DiscountAmount_USD]/InternetSales_USD[OrderQuantity]))