Re: st: Subtracting in Variable by a Group

[Nick Cox <njcoxstata@gmail.com>]

Your do-file could consist of

bysort group (date): gen time = date - date[_n-1]

but many Stata users would just type that one command.


Nick

On 29 Mar 2013, at 16:10, Arya Eskamani <Arya.Eskamani@assist-rx.com>  
wrote:
> Hello and thank you in advance
>
> I'm attempting to find the difference in days between events within  
> different groups.  The date is formatted %td (daily). Here's an  
> example of my dataset:
> group    event    date
> A    1    08mar2013
> A    2    10mar2013
> A    3    15mar2013
> B    1    10sep2012
> B    2    15sep2012
> C    1    07aug2011
> C    2    23aug2011
> C    3    23aug2011
> C    4    24aug2011
> D    1    25dec2012
> E    1    01jan2013
>
> I want to generate a new variable, say "time" , that calculates the  
> difference in days from one event occurring to the next event in  
> between the groups.  For example, in group A there are 2 days  
> between even 1 and event 2 and then 5 days from event 2 to event 3,  
> and similarly for others.  The example below shows what I'm trying  
> to achieve:
> group    event    date    time
> A    1    08mar2013    .
> A    2    10mar2013    2
> A    3    15mar2013    5
> B    1    10sep2012    .
> B    2    15sep2012    5
> C    1    07aug2011    .
> C    2    23aug2011    16
> C    3    23aug2011    0
> C    4    24aug2011    1
> D    1    25dec2012    .
> E    1    01jan2013    .
>
> Notice that the time variable is blank for 1st events because there  
> is no previous event to subtract from.  I've never ran an operation  
> within a column before and have been doing this in excel but would  
> like to just write a do file on this.  Any help is much  
> appreciated.  Thank you again.
>
> Arya Eskamani
>
>
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