st: Difference in values within the same variable

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

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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