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st: RE: Re: corr() with weights, -by-

From   "Nick Cox" <>
To   <>
Subject   st: RE: Re: corr() with weights, -by-
Date   Sun, 8 Jun 2008 16:37:40 +0100

On the other hand, if you want the covariances alongside the data, this
approach is cleaner than recourse to -levelsof-: 

gen cov = . 
egen group = group(country year) 
su group, meanonly 

quietly forval i = 1/`r(max)' { 
	corr x l2.x [aw=weight] if group == `i', cov 
	replace cov = r(cov_12) if group == `i' 

See also 

FAQ     . . . . . . . . . . Making foreach go through all values of a
        8/05    Is there a way to tell Stata to try all values of a
                particular variable in a foreach statement without
                specifying them?

However, -statsby- and then using -merge- may be just as efficient. 


Michael Blasnik

You  want the -statsby- command.  Something like this work:

statsby cov=r(cov_12), by(year country) : corr x l2.x  [aw= weight], cov

Philipp Rehm

> I am looking for an efficient way to extract weighted covariances, for
> by-able problem.
> Currently, I am running a very inefficient loop (using a lot of 
> -levelof-s and if-qualifiers), along these lines (my data is -tsset-):
> corr x l2.x [aweight=weight] if year==`i' & country==`j', covariance
> I then write the results [r(cov_12)] into a variable, again using 
> if-qualifiers.
> A much quicker way to do this is to use the corr() function (written
> Nick Winter) from the egenmore package. It does exactly what I want, 
> except that it does not accept weights.
> I can also run something like:
> bys country year: correlate(x l2.x) [aweight=weight], covariance
> but r(C) leaves behind the covariance matrix only for the very last 
> estimation.
> So, ideally I am looking for a version of Nick Winter's corr() which 
> accepts weights.
> Any ideas / suggestions would be appreciated. I prefer a solution that

> works in Stata 9, but I do have access to Stata 10.

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