Mean of observations in a series for each specified group defined by distinct values.
Syntax:		@meansby(x, y[y1, y2, ... yn, s])
	x:	series
	y1...yn	series, alpha, group
	s:	(optional) sample string or object 
Return:		series
Compute the mean of observations in x for group identifiers defined by distinct values of y, using the current or specified workfile sample.
Examples
show @meansby(x, g1, g2)
produces a linked series of by-group means of the series x, where members of the same group have identical values for both g1 and g2.
Cross-references