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RE: st: First occurence


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   RE: st: First occurence
Date   Thu, 17 May 2007 22:52:53 +0100

Ho hum. I don't think Austin meant what he typed. 
But I agree with his strong signal that you should 
explain your data set-up clearly. 

The first year any id appears can be tagged by something like 

bysort bdsmaster (year) : gen first = _n == 1 

assuming that -bdsmaster- is an id code. 

Austin's equivalent is 

egen minyr = min(year), by(bdsmaster) 
gen first = year == minyr 

but you can get there in one line. 

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

Austin Nichols replied to Meryle Weinstein 

> It's always advisable to frame your question in terms of data everyone
> can use, e.g. by starting your example with -sysuse auto- and showing
> the result you want.  That said, one way (among many) to generate an
> indicator for first year is:
> 
> egen minyr=min(year), by(bdsmaster)
> gen fyr=(year==fyr)
> 
> > I've tried following the syntax in FAQ and archives but am 
> having difficulty
> > with it.  I have a dataset that has an idcode and a year 
> for a number of
> > schools.  I want to identify the first year that the idcode 
> appears.  I've
> > tried the following but still don't' get the first occurrence.
> >
> > by bdsmaster (year), sort: gen noccur = sum(bdsmaster)
> > by bdsmaster: gen byte first = noccur == 1  & noccur[_n - 
> 1] != noccur
> >
> > gen byte first = sum(bdsmaster == 1) == 1  & 
> sum(bdsmaster[_n - 1] == 1) ==
> > 0

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