STDDEV_POP function [Aggregate]

Computes the standard deviation of a population consisting of a numeric-expression, as a DOUBLE.

STDDEV_POP ( [ ALL ] expression )

The following statement lists the average and variance in the number of items per order in different time periods:

SELECT year( ship_date ) AS Year, quarter( ship_date )
  AS Quarter, AVG( quantity ) AS Average, 
  STDDEV_POP ( quantity ) AS Variance 
FROM SalesOrderItems GROUP BY Year, Quarter 
  ORDER BY Year, Quarter;

Computes the population standard deviation of the provided value expression evaluated for each row of the group or partition (if DISTINCT was specified, then each row that remains after duplicates have been eliminated), defined as the square root of the population variance.