04-16-2015 04:23 PM
I have a question about how to calculate day difference from the first date within a group (ID). Thanks much for your answer.
My data looks like as below. There are two columns (ID, date) and some other columns, which I did not show here but I need keep. I need create a new variable date_diff to show day difference from the first date within each ID. I showed a example of ID=1 as below for how I need calculate.
ID date date_diff
1 06JUL2006 0
1 23JUL2006 23JUL2006 - 06JUL2006
1 08AUG2007 08AUG2007 - 06JUL2006
04-16-2015 04:33 PM
Assuming your data are sorted properly, by ID DATE, here's an elegant way:
date_diff + dif(date);
if first.id then date_diff=0;
(Of course, beauty is in the eye of the beholder.) Good luck.