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Re: st: linear interpolation
i think i overdid my example. in your case the following should work:
tsset (your vars)
and if you want to extrapolate to 2005, simply type -edit-, add only
one observation (2005 for the year variable) and repeat as above.
anyway, to show you the underlying logic of my previous post, let's say
you have 5,000 obs total and you need to add 18 new observations to
each group identified by state and year. the "manual" way is to type
-edit-, go to the very end of your dataset and add the years that you
want, e.g. 1981, 1982, ..., 1989, 1991, 1992, ..., 1999, then -tsset-
the data and use -tsfill, full-. this works well when you have only
few observations to add (as in your case).
my original answer was better suited to adding a large number of years
to each group. instead of the "manual" editing, you add a "fake" panel
with all the years you want to use (e.g. 1980-2000 or 21 years total)
local N = `r(N)' + 21
set obs `N'
now you will have 5,021 obs, and you want to replace observation 5,001
with 1980, 5,002 with 1981, ..., 5,021 with 2000. so in this articular
case, you have to use
replace year = -3021 + _n if year==.
and then to -tsset- and -tsfill, full-.
but again, as i have just discovered, stata expands the in-between
years automatically so there's no need to follow either of the long
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