re:st: Difference in values within the same variable

[Christopher Baum <kit.baum@bc.edu>]

<>
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
-----------------


Kit Baum   |   Boston College Economics & DIW Berlin   |   http://ideas.repec.org/e/pba1.html
                             An Introduction to Stata Programming  |   http://www.stata-press.com/books/isp.html
  An Introduction to Modern Econometrics Using Stata  |   http://www.stata-press.com/books/imeus.html


*
*   For searches and help try:
*   http://www.stata.com/help.cgi?search
*   http://www.stata.com/support/statalist/faq
*   http://www.ats.ucla.edu/stat/stata/