st: re: date and if function

[Kit Baum <baum@bc.edu>]

Ziad said

I want to calculate the number of days elapsed between date of
patients visits to the clinic, patients are expected to come each 4th
week, i.e. week4, week8, week12 etc..
I have date of visit (var date) and dummy variable for weeks, i.e.
week4 (1 or 0), week8 (1 or 0)

I am trying to generate new variable to calculate n of days between
week8 and week4, by saying
dayat8 = (date if week4==1) - (date if week8==1)


The last line does not vaguely resemble valid Stata syntax. But you  
could say something like

generate dayat8 = date * week4 - date * week8

I doubt if that's what you really want to do, though.

If you converted the data to the long form with -reshape- (as is  
often suggested on this list) you would end up with something like

patientid week date
111            4      06jun2007
111           8       05jul2007

etc.  and you could then calculate what you want as

bysort patientid (week): gen interval = date - date[_n-1]

which would return missing for the first record and the number of  
days between those dates for the second. You could then 'reshape  
wide' to return the data to the wide form.

Computations such as these are usually easier to perform in the long  
form.

Kit

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


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