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
[email protected]
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..
*
* 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/