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st: Negative deff values in survey analysis


From   Ángel Rodríguez Laso <angelrlaso@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   st: Negative deff values in survey analysis
Date   Wed, 3 Jun 2009 10:21:40 +0200

Dear Statalisters,

I'm confused with the following results:



. svyset psu [pweight=weight2007], strata(healtharea)fpc(psusperhealtharea)

      pweight: weight2007
          VCE: linearized
     Strata 1: healtharea
         SU 1: psu
        FPC 1: psusperhealtharea

.
end of do-file

. svy: tab p29, deff deft
(running tabulate on estimation sample)

Number of strata   =        11                  Number of obs      =     12140
Number of PSUs     =      1266                  Population size    = 12134,139
                                                Design df          =      1255

-------------------------------------------------
Any permanent
disability | proportions         deff         deft
----------+--------------------------------------
    0, no |       ,8887        -1981        ,9783
    1, yes |       ,1113        -1981        ,9783
          |
    Total |           1
-------------------------------------------------
  Key:  proportions  =  cell proportions
        deff         =  deff for variances of cell proportions
        deft         =  deft for variances of cell proportions




Why do I get large negative deff values? Deft resembles more what I
was expecting, but it should be the square root of deff and obviously
this is not the case. Do you have any explanation for these results?

Many thanks,

Angel Rodriguez-Laso
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