st: How to generate lags where each variable to be lagged has multiple values in the previous time periods

[Stuart Buck <stuartbuck@gmail.com>]

Here's what I have:

Passage rates for all Texas schools for 2008, 2009, 2010, and 2011 --
this is important -- by grade. So each row in the dataset is School,
Year, Grade, and then scores (plus other demographic variables, etc.).

In other words, the dataset looks like this:

Year     SchoolID     Grade     TestScore
2011    1                  6               ***
2011     1                 7               ***
2011     1                 8               ***

And so on and so forth -- multiple grades in each school in each year.

Here's what I want:

To be able to regress any given school's performance in Grade X in
Year T on, among other things, how that same school did with the same
cohort of kids in the previous grade (Grade X-1) in the previous year
(Year T-1). I.e., if a middle school's Grade 8 passage rate in 2011 is
the outcome, I'd like to be able to control for that same school's
Grade 7 passage rate in 2010, thus giving a somewhat crude measure of
how much that group of kids progressed since the previous year.

How would I generate an all-purpose lagged TestScore variable for all
the schools in the dataset, lagging by both year and grade at once?
All the Stata instructional material I see on lagged variables just
lags based on time, not on both time and some other variable too
(grade).

thanks,

--
Stuart Buck
http://about.me/stuartbuck
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