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st: RE: RE: check if variable is constant within id


From   "Mak, Timothy" <Timothy.Mak@iop.kcl.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: RE: check if variable is constant within id
Date   Wed, 16 May 2007 14:21:59 +0100

Thank you Nick. 

Tim 

PS The FAQ page did not mention the -assert- command, which I didn't
know before.  

-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nick Cox
Sent: 16 May 2007 13:46
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: check if variable is constant within id

I am pleased to hear that you don't want a program to do this. 

First without missings, 

bysort id (var1) : assert var1[1] == var1[_N] 

checks that values of var1 are constant within id. 

Second with missings, we need to set missings on one side: 

gen byte ismiss = missing(var1)
bysort ismiss id (var) : assert var1[1] == var1[_N] 

That's two lines. Perhaps someone can shorten it. 

But the solutions here can be worked out from this FAQ, which says more
that I won't repeat here: 

How do I list observations in a group that differ on a variable? 
http://www.stata.com/support/faqs/data/diff.html

My suggestion to you is to scan the FAQs before posting.  

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

Mak, Timothy
 
> I'm looking for an easy way to check if all the observations for a 
> particular variable (say var1) within id (a subject identifying
> variable) are constant a) for the case where missings can be excluded,

> and b) for the case where missings cannot be excluded. The best answer

> would be a one-liner, with a short display. I don't want a program to 
> do this.

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