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st: RE: cumulative moving sum when there are repeated date values


From   "Nick Cox" <n.j.cox@durham.ac.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: RE: cumulative moving sum when there are repeated date values
Date   Thu, 5 Feb 2009 10:49:42 -0000

This is discussed in 

SJ-7-3  pr0033  . . . . . . . . . . . . . .  Stata tip 51: Events in
intervals
        . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .  N.
J. Cox
        Q3/07   SJ 7(3):440--443                                 (no
commands)
        tip for counting or summarizing irregularly spaced
        events in intervals

Nick 
n.j.cox@durham.ac.uk 

Padma Rao Sahib

I have a panel dataset of acquisition attempts made by firms in which I 
would like to calculate the cumulative number of acquisitions a firm 
completes in the past 5 years prior to
the current acquisition.
Firms enter the dataset if they make an acquisition attempt but as some 
acquisitions are later abandoned, I have an acquisition completion dummy

"acdum" that is 1 if the
acquisition is completed, 0 otherwise" .

My data looks like this:
Acdum is the Acquisition completion dummy.

Firmid Date Acdum Cumulative

Shell 24 Jan 1993 1 0

Shell 1 Dec 1995 0 1

Shell 3 Aug 1996 1 1

Shell 3 Aug 1996 1 2

Mittal Steel 3 Jan 1993 1 0

Mittal Steel 4 March 2000 1 0

Mittal Steel 5 April 2001 1 1


I would like to count the cumulative number of acquisitions made by the 
firm in the past 5 years prior to the current acquisition (even if they 
occured in the same year ).
By searching Statalist, I found a solution using " tssmooth, egen and a 
filter. However, these solutions involving tsseting the data and as it 
can be seen from the example above,
I have firms that sometimes announce two acquisitions as being completed

on the same day (and I don't have any finer time details like hours 
etc.) to tsset the data properly.
As a result I get an error message saying that I have repeated time 
values within my panel. The actual ordering of acquisitions that occur 
on the same day does not matter,
I simply need to sum up the number of acquisitions prior to the current 
one as long as they occurred in a 5 year interval, and it is OK if the 
ones on the same day are
off by one acquisition.

I tried something like this:
bysort firmid (date):gen prevacq = sum(acdum)
by firmid: gen prevacq5 = prevacq - prevacq[_n-4)

(also borrowed and adapted from a solution to a related question on 
statalist) but this
does not quite work and seems to cap the number of previous acquisitions

at 4.

Out of desperation, I thought of manually adding a day to the date 
announced as it is the year that is important
but there are too many observations for this to be feasible...I think I 
need a loop of some sort but my STATA loop knowledge is somewhat
deficient..


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