Re: st: Difference in values within the same variable

[Sara Muller <s.muller@cphc.keele.ac.uk>]

Thank you for everyone's helpful comments. I can now identify the people 
I need in my analysis sample.
Very best wishes,
Sara

On 03/01/2012 14:28, Nick Cox wrote:
> A small extra flourish would be
>
> bysort patid (timesince) : gen selected = timesince[1]<  183
> edit if selected
>
> where "183" or any similar value represents six months in days. That
> is, if the smallest value of -timesince- is less than six months, then
> that patient had two or more prescriptions in a period of six months
> or less. Such patients have -selected- 1 and others have -selected- 0.
>
> Nick
>
> On Tue, Jan 3, 2012 at 2:06 PM, Christopher Baum<kit.baum@bc.edu>  wrote:
> > <>
> > Sara said
> >
> > I have long data with multiple prescriptions per person. I want to
> > define the length of time between successive prescriptions (see data
> > below, which is sorted by patid (person ID) and presc_date (date of
> > prescription)), with the eventual aim of including in my analysis only
> > people who received 2 or more prescription within 6 months of each other.
> >
> > I'm not bothered really how I do this, although I have been trying to do
> > it by calculating another variable that gives the time from the previous
> > prescription (within patid). I believe the equivalent command in SPSS is
> > called a 'lag', but searching  for this in Stata doesn't see to have
> > turned up the right stuff.
> >
> > --------------
> > clear all
> > input patid  str9  presc_date
> > 4457    11aug1997
> > 4457    18aug1997
> > 4457    15sep1997
> > 4457    29sep1997
> > 4457    27oct1997
> > 4457    15dec1997
> > 4457    15dec1997
> > 4457    16apr1998
> > 4457    11may1998
> > end
> > g rxd = date(presc_date,"DMY")
> > format rxd %td
> > bysort patid (rxd): g timesince = rxd[_n] - rxd[_n - 1]
> > list
> > -----------------
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