st: Friends' characteristics

[Chris Ruebeck <ruebeckc@lafayette.edu>]

(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.)

Thanks!
Chris


Christopher S. Ruebeck, Ph.D.
Assistant Professor
Economics & Business
Lafayette College
Easton, PA 18042
610/330-5309, fax 610/330-5715
ruebeckc@lafayette.edu
http://www.lafayette.edu/~ruebeckc/


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