program define _gcorr
*! v1.2 gmh jun 5 2001
			gettoken type 0 : 0
			gettoken vn   0 : 0
			gettoken eqs  0 : 0  

	syntax varlist(min=2 max=2) [if] [in] [, BY(string)]

if `"`by'"' ~="" {
	local by = "by(`by') "
	}

gettoken var1 varlist:varlist
gettoken var2 varlist:varlist


tempvar cross m1 m2 touse cnt sd1 sd2
mark `touse' `if' `in'
markout `touse' `var1' `var2'
	qui egen `type' `m1'=mean(`var1') if `touse' , `by'
	qui egen `type' `m2'=mean(`var2') if `touse', `by'
	qui egen `type' `cross'=mean(`var1'* `var2') if `touse', `by'
	qui egen `cnt'=sum(`touse'), `by'
	qui egen `type' `sd1'=sd(`var1') if `touse', `by'
	qui egen `type' `sd2'=sd(`var2') if `touse', `by'
* dof correction
	qui gen `type' `vn'= `cnt'/(`cnt'-1)* (`cross'-`m1'*`m2')/(`sd1'*`sd2')
end