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st: Lincom after devcon, and another minor devcon question


From   "Wafa Hakim Orman" <wafa1024@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Lincom after devcon, and another minor devcon question
Date   Wed, 3 Oct 2007 11:31:26 -0500

Hi,

I am estimating a model using survey data. Since my model has a lot of
dummy variables, for which I need exact estimates, I am using devcon
after my survey regression. I then need to test a linear combination
of the coefficients. When I use lincom, I get an error saying: "matrix
has missing values." On the other hand, the test and nlcom commands
work just fine (so I can actually use nlcom to obtain the linear
estimates I need).

This is basically what I'm doing (in short):

svyreg lnincome age agesq female married S1 S2 S3

(where S* are dummy variables & S0 is the left-out reference group.)

devcon, groups(female male, married unmarried, S0 S1 S2 S3)

Now if I do:
test S0 + S1 - S2 - S3 = 0
or
nlcom (_b[S0] + _b[S1] - _b[S2] - _b[S3])

I get an answer with no issues. But if I do:
lincom S0 + S1 - S2 - S3
I get the error saying "matrix has missing values." Lincom works fine
after the regular estimation without using devcon. What exactly is
going on, & why this difference? Aren't lincom and nlcom both
estimating t-statistics? Is this a known issue or bug, or is there
something wrong with what I'm doing?

(Since I can get the estimates I need using nlcom, this is hardly a
pressing matter, but I thought I'd bring it to people's attention just
in case.)


My other minor devcon question: what is the best way to use devcon
with an interaction effect between two dummy variables (other than
generating all the 4 possible cases and using those)? In my case, this
would be the interaction: married*female.


Thanks a ton!


Wafa.
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