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Re: st: RE: Unique Case ID in Large Panel


From   smerryman@kc.rr.com
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: RE: Unique Case ID in Large Panel
Date   Wed, 04 Aug 2004 12:49:53 -0500

I have no solution -- just a note. Both -tsset- and -xtdes- check for repeated time values within a panel, though they use different methods.

-xtdes- performs:

by id time: assert _N== 1

while -tsset- performs:
by company year: gen diff = year[_n+1] - year[_n]
sum diff, meanonly

and then checks that the minimum is zero.

I am at a loss to think of a situation where the minimum time difference is not zero yet you would have repeated time values with in a panel.

What message do you get after -tsset- ?


Scott

----- Original Message -----

> > 
> > My problem is that -xtdes- says my two key variables do not 
> together 
> > uniquely identify the cases in my large panel data set, but 
> > both the Stata 
> > code I wrote to identify the duplicates, and SPSS's 
> > automatically generated 
> > syntax to "find duplicates" (operating on a precursor data 
> > set with the 
> > same cases and variables) say there are no duplicates.
> > 
> > Searching the FAQs on "xtdes" produced no hits.
> > 
> > Specifics:
> > 
> > 1) I have 1,322,511 cases.   The data set is about 700 MB.  
> Memory 
> > allocated is 1.4GB (and wasn't it a kick in the fatoozle to 
> > discover that's 
> > all WinXP will let me allocate from the 4 GB on my system!).
> > 
> > 2) I first did -tsset- using DRVNUM (employee id) and CDATE 
> > (paycheck date).
> > 
> > 3) -xtdes- message:
> > 
> >    DRVNUM:       10001, 10003, ..., 99051                        
>  n 
> > =      28946
> >    CDATE:        15344, 15351, ..., 16072                        
>  T 
> > =        105
> >             Delta(CDATE) = 7; (16072-15344)/7 + 1 = 105
> >             (DRVNUM*CDATE does not uniquely identify observations)


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