Re: st: Difference in values within the same variable

[A Loumiotis <antonis.loumiotis@gmail.com>]

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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> *   http://www.ats.ucla.edu/stat/stata/

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