VAR_POP function [Aggregate]

Computes the statistical variance of a population consisting of a numeric-expression, as a DOUBLE.

VAR_POP ( [ ALL ] expression )

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

SELECT year( ShipDate ) AS Year, quarter( ShipDate )
  AS Quarter, AVG( Quantity ) AS Average, 
  VAR_POP( Quantity ) AS Variance 
FROM SalesOrderItems GROUP BY Year, Quarter 
  ORDER BY Year, Quarter

Computes the population variance 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 sum of squares of the difference of value expression, from the mean of value expression, divided by the number of rows (remaining) in the group or partition.

Population-based variances are computed according to the following formula: