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st: RE: -group- within varlist


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: -group- within varlist
Date   Mon, 23 Jan 2006 23:48:42 -0000

You have been on this list for some time and 
asked many questions, but it doesn't seem 
that you are making much use of the help 
facilities, including -search- and the FAQs. 

I am not clear what the precise underlying question is 
here, but the answer is definitely Yes. 
There are many alternatives. 

Much depends on what you mean by identifier. I 
take that to mean ideally that each observation 
can be identified uniquely. If no such variable 
exists, it can always be created with 

. gen long id = _n 

For much more discussion, see

FAQ     . . . . . . . . . . . . . . . . . . . . . . Creating group identifiers
        3/01    How do I create individual identifiers numbered
                from 1 upwards?
                http://www.stata.com/support/faqs/data/group.html

As far as -egen, group()- is concerned, -by:- indeed can't be 
used with it. But that functionality is completely 
unnecessary. If -by:- were allowed it would just 
act if it were the first variable you specified. So
whatever variable you want as a -by:- argument 
should be the first named. So instead of 

. egen group = group(b a), by(c) 

or 

. by c: egen group = group(b a) 

you just go 

. egen group = group(c b a) 

But -group()- generates grouping variables, which in 
general are not identifiers. 

Either way: -egen, group()- is just an application of -by:- 
in any case. For more control in this and many other problems, 
you need to master -by:- directly. There is a tutorial at 

SJ-2-1  pr0004  . . . . . . . . . . Speaking Stata:  How to move step by: step
        Q1/02   SJ 2(1):86-102                                   (no commands)
        explains the use of the by varlist : construct to tackle
        a variety of problems with group structure, ranging from
        simple calculations for each of several groups to more
        advanced manipulations that use the built-in _n and _N

Nick 
n.j.cox@durham.ac.uk 

Terra Curtis
 
> Since the -egen- function -group- cannot be combined with 
> "by", does anyone
> know if there is an alternative method (or Stata function) 
> for creating an
> ID within a varlist?
> 
> PS - I am using Stata 8. 

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