Re: st: Friends' characteristics

[Chris Ruebeck <ruebeckc@lafayette.edu>]

Thanks!  I see the key is using -rename- and -merge- .

Chris


On Aug 31, 2006, at 10:06 AM, Stas Kolenikov wrote:

> I would go with a merge, something like
>
> tempfile friend1 friend2 friend3 friend4
>
> preserve
> keep id gpa
> rename id friend
> forvalues k=1/4 {
>   rename friend friend`k'
>   rename gpa gpa_f`k'
>   // note that the mask friend will be matched to friend1 when  
> k==2, etc.
>  sort id`k'
>  save `friend`k''
> }
> restore
>
> forvalues k=1/4 {
>   sort friend`k'
>   merge friend`k' using `friend`k''
> }
>
> egen peer_gpa = rmean(gpa_f*)
>
> Of course I have not tried it working, but it should give you an idea.
> I don't know if it is going to be much faster (and it very well might
> be), but it is also somewhat clearer, I think.
>
> On 8/30/06, Chris Ruebeck <ruebeckc@lafayette.edu> wrote:
>
> > (Previously sent but didn't see it appear on Statalist.)
> >
> > Suppose my data set has these 6 variables,
> >
> >         id : this respondent's ID,
> >         gpa : this respondent's GPA, and
> >         friend1-4 : the IDs (possibly missing) of this  
> > respondent's friends.
> >
> > I would like to create four new variables that record the GPA of each
> > respondent's friends, and then take their average.  I have many
> > observations and want to avoid slower methods.  Here is my code for
> > the first friend.
> >
> > gen gpaf1 = .
> > egen group = group(friend1)
> > summarize group, meanonly
> > foreach num 1 / `r(max)' {
> >         summarize id if group==`num', meanonly
> >         local idf = r(mean)
> >         summarize gpa if id==`idf', meanonly
> >         replace gpaf1 = r(mean) if group==`num'
> > }
> >
> > I figure I can nest this in a forvalues loop from 1-4, and then use -
> > egen ... rowmean(gpaf1-4)- to get the mean over friends.  In the code
> > above, levelsof could replace the -egen ... group(friend1)- but macro
> > length limits would require splitting the friends' ids into two to
> > four groups.
> >
> > Is there a faster method, perhaps with Mata?
> >
> > (An additional wrinkle: some friends may no longer be in the
> > database---so an observation's friend1, for example, may contain a
> > number that is not the id of any observation.  I think the code above
> > is robust to that problem, but perhaps this is another potential
> > speed improvement.)
> --
> Stas Kolenikov
> http://stas.kolenikov.name
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