st: how to generate sum of distinct id1, by id2, in the last n years

["Pierre Azoulay" <pierre.azoulay@gmail.com>]

Dear Statalisters,

I have what I believe a simple programming question that I can't quite solve.
I have a panel of dyads, where each member of the dyad is a coauthor.
Each dyad is composed or a "superstar" and a "simple joe/jane."

For instance:

star_id		id		year		nbpapers
---------------------------------------------------------
1		2		1972		1
1		2		1973		0
1		2		1974		2
1		2		1975		3
1		2		1976		0
1		2		1977		4
1		3		1970		1
1		3		1971		0
1		3		1972		0
1		3		1973		2
1		4		1978		2
1		4		1979		1
1		5		1977		4
1		5		1978		1
1		5		1979		0
1		5		1980		1
1		5		1981		1

So superstar #1 has 4 "simple joe collaborators" numbered 2,3,4, and 5.
In each year, the data records how many publications exist for
superstar i and simple joe/jane j.


I would like to collapse this data at the superstar/year level and
create a variable that records the sum of distinct "simple joes" in
the last 3 years.
In other words, I'd like to create the variable stk_nbcoauth_it that is:

star_id	year	stk_nbcoauth_it
---------------------------------
1	1970	1
1	1971	1
1	1972	2
1	1973	2
1	1974	2
1	1975	2
1	1976	1
1	1977	2
1	1978	3
1	1979	3
1	1980	2
1	1981	2

I have fiddle with bysort star_id id (year), but without clear
success. Could anyone help?

Regards,

Pierre



--------------------------------------------------------------------
Pierre Azoulay
Assistant Professor of Strategy
Massachusetts Institute of Technology
Sloan School of Management
50 Memorial Drive — E52-555
Cambridge, MA 02142-1947

Tel [Sloan]: (617) 258-9766
Tel [NBER]: (617) 588-1464
Fax: (617) 253-2660

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