st: Variable running totals

["Schaffer, Mark E" <M.E.Schaffer@hw.ac.uk>]

Hi all.  "Variable running totals" isn't the best description of the
problem, but it's not too far off.

A colleague has written to me with the following problem.  He has a
panel dataset with two variables: id and date.  (He has some other
variables but those are the two that matter.)  There may be multiple
observations on id for a given date.  The date variable is in Stata %td
format (#days after 01jan1960).  So it looks like this:

id	date
1	1000
1	1002
1	1002
1	1200
1	1250
2	1050
2	1059
2	1085

...etc.


The question is, how to construct a variable that counts the number of
observations that an individual (id) appears in the dataset up to 30
days previously.  If we call the variable count30, it would look like
this:

id	date	count30
1	1000		1
1	1002		2
1	1002		3
1	1200		1
1	1250		1
2	1050		1
2	1059		2
2	1085		2

...etc.

I suspect there's an easy way of doing this, but the only ways I could
think of involved brute force looping through observations.

Any ideas?

--Mark


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