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Re: st: Difference in values within the same variable


From   Nick Cox <njcoxstata@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Difference in values within the same variable
Date   Tue, 3 Jan 2012 14:33:19 +0000

This won't work without extra care. Prescription dates occur
irregularly, so any -tsset- or -xtset- would need to be  done using a
specially created sequence variable that took successive integer
values for any patient. I concur with Kit Baum in recommending working
with [_n] and [_n-1] rather than time series operators.

Nick

On Tue, Jan 3, 2012 at 2:11 PM, A Loumiotis <antonis.loumiotis@gmail.com> wrote:
> Hi Sara,
> You can use:
>
> gen passtime = presc_date - l.presc_date
>
> assuming your dataset is already declared as panel using -xtset- or -tsset-.
>
> Type "help tsvarlist" for help on time series operators.
>
> Antonis
>
> On Tue, Jan 3, 2012 at 3:19 PM, Sara Muller <s.muller@cphc.keele.ac.uk> wrote:
>> Hi,
>>
>> I'm sorry if this is a silly question. I have searched in the help and
>> online, but I don't know what to look up.
>>
>> 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.
>>
>> If anyone could point me in the right direction, I'd be very grateful.
>>
>> Best wishes,
>> Sara
>>
>> patid    presc_date
>> 4457    11aug1997
>> 4457    18aug1997
>> 4457    15sep1997
>> 4457    29sep1997
>> 4457    27oct1997
>> 4457    15dec1997
>> 4457    15dec1997
>> 4457    16apr1998
>> 4457    11may1998
>>

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