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st: predictive margins not estimable in svy


From   susan@hailpern.com
To   statalist@hsphsun2.harvard.edu
Subject   st: predictive margins not estimable in svy
Date   Fri, 07 Aug 2009 15:52:47 -0700

Hello,

I am having problems using the Stata 10 "margins" command with survey
data.  

I have a created a subpop of ~2500 observations (total 5000+) and would
like to obtain the age- sex- race adjusted prevalence of my dichotomous
outcome by cystatin-c quartile. Data is svyset; svy logistic model
including age, sex and race and cystatin-c was run, followed by the
marginals command.  

The output says (not estimable). (below)

Can anyone help?  I do get estimations if I take it out of survey mode,
Thanks,
Sue

. svy, subpop(subpopnodm): logistic pad i.x age56 sex01 race2
(running logistic on estimation sample)

Survey: Logistic regression

Number of strata   =        28                 Number of obs      =     
 5684
Number of PSUs     =        57                 Population size    = 
230532837
                                               Subpop. no. of obs =     
 2472
                                               Subpop. size       =
90812359.8
                                               Design df          =     
   29
                                               F(   6,     24)    =     
28.73
                                               Prob > F           =    
0.0000

------------------------------------------------------------------------------
             |             Linearized
         pad | Odds Ratio   Std. Err.      t    P>|t|     [95% Conf.
Interval]
-------------+----------------------------------------------------------------
           x |
          2  |   1.521746   .5364044     1.19   0.243     .7400256    
3.12923
          3  |   4.057542   2.135947     2.66   0.013     1.382564   
11.90805
          4  |   9.510348   3.931692     5.45   0.000     4.083106   
22.15145
             |
       age56 |   4.655526   2.164545     3.31   0.003     1.798843   
12.04881
       sex01 |    1.14324    .236113     0.65   0.522      .749361   
1.744149
       race2 |   2.314983   .5035135     3.86   0.001     1.483732   
3.611936
------------------------------------------------------------------------------

. margins i.x , subpop(if subpopnodm==1)

Predictive margins                                Number of obs   =     
 3009
                                                  Subpop. no. obs =     
 2472
Model VCE    : Linearized

Expression   : Pr(pad), predict()

------------------------------------------------------------------------------
             |            Delta-method
             |     Margin   Std. Err.      z    P>|z|     [95% Conf.
Interval]
-------------+----------------------------------------------------------------
           x |
          1  |  (not estimable)
          2  |  (not estimable)
          3  |  (not estimable)
          4  |  (not estimable)
------------------------------------------------------------------------------



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