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Re: st: stumped with lincom after svymean


From   jpitblado@stata.com (Jeff Pitblado, StataCorp LP)
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: stumped with lincom after svymean
Date   Tue, 31 Aug 2004 11:16:56 -0500

"Robert Bozick" <robertbozick@hotmail.com> asks about using -test- after
-svymean, by()-:

> Here is what I am trying to accomplish.  I have a continuous variable y, and 
> a categorical variable x.  The categorical variable x has three categories: 
> 0, 1, and 2.  I want to test whether y is different among the categories of 
> my variable x.  My data are collected using a stratfied, cluster design and 
> therefore I must use the survey commands.
> 
> In previous versions of stata, I would use the code:
> 
> svymean y, by(x)
> svytest [x]0=[x]1
> 
> Stata 8 says that svytest no longer exists and that I should use the lincom 
> command.  Maybe the coffee hasn't set in, but I cannot figure out how to do 
> replicate the above code using the lincom command based on the information 
> from Stata's help screen.
> 
> So my question is two-fold:
> 1 - How do I test group differences using the svy command in Stata 8 based 
> on my above example?
> 2 - Is it possible to estimate a one-way anova to detect group differences 
> with svy commands in Stata 8 based on my above example?

Here is an example using the auto data (not that I remove the labels so that
my example appears closer the one above):

	. sysuse auto, clear
	. label drop _all
	. svyset, srs
	. svymean mpg, by(foreign)
	. test [mpg]0 = [mpg]1

When the -by()- option is supplied to -svymean-, the name of the variable that
we are taking the mean of becomes something like an equation identifier when
used with -test-.  The values of the categories in the -by()- variable are
used to identify which mean we are talking about.

Here is the output from the above example:

***** BEGIN:
. sysuse auto, clear
(1978 Automobile Data)
                                                                                
. label drop _all
                                                                                
. svyset, srs
simple random sample (SRS)
                                                                                
. svymean mpg, by(foreign)
                                                                                
Survey mean estimation
                                                                                
pweight:  <none>                                  Number of obs    =        74
Strata:   <one>                                   Number of strata =         1
PSU:      <observations>                          Number of PSUs   =        74
                                                  Population size  =        74
                                                                                
------------------------------------------------------------------------------
Mean   Subpop. |   Estimate    Std. Err.   [95% Conf. Interval]        Deff
---------------+--------------------------------------------------------------
mpg            |
    foreign==0 |   19.82692    .6558681    18.51978    21.13407           1
    foreign==1 |   24.77273    1.386503    22.00943    27.53602           1
------------------------------------------------------------------------------
                                                                                
. test [mpg]0 = [mpg]1
                                                                                
Adjusted Wald test
                                                                                
 ( 1)  [mpg]0 - [mpg]1 = 0
                                                                                
       F(  1,    73) =   10.40
            Prob > F =    0.0019
***** END:

--Jeff
jpitblado@stata.com
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