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Re: st: base category in margins output


From   Eduardo Nunez <enunezb@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: base category in margins output
Date   Wed, 19 Oct 2011 10:30:16 -0400

Richard,
Excelent presentation!
I couldn't, however, get a clue to my question.
The main difference I have with the examples you showed in your
presentation under adjusted predictions is that the default prediction
from a logistic regression is pr; after Cox, is hazard ratio.
Thus, there must be a base group from which the estimate refers to.
Running the univariate Cox, I got the base categories in the results
of the margins as 1 1....1 (as shown below)
Meaning category 1 for urea_2_ca35 & value "0" for LED_sqrt

stcox i.urea_2_ca35##c.LED_sqrt, efron
margins, at(LED_sqrt=(0(3)21) (asobserved) _all) over(urea_2_ca35) post

---------------------------------------------------------------------------------
                |            Delta-method
                |     Margin   Std. Err.      z    P>|z|     [95%
Conf. Interval]
----------------+----------------------------------------------------------------
_at#urea_2_ca35 |
           1 1  |          1          .        .       .            .
         .
           1 2  |   .8365883   .4008474     2.09   0.037     .0509418
  1.622235
           1 3  |   .3564519   .1606048     2.22   0.026     .0416723
  .6712315
more output not shown............

When I run the adjusted Cox model, the prior base categories 1 1 now
reports 1.44 (see below)

stcox Iedad__1 sexo dm_i obesidad_i primer_ingreso_i portador_de_dai
pad iam demencia nyha3_4 efLT50##c.sbp_i acxfa##c.fc_i br_i
edemas_tm_i derrame_pleural_i ///
Ina_i__1 Ina_i__2 Iac_u__1 Ibnp___1 Ipcr___1 Icole__1 bbloq_previo_i
bbloq estatina nitratos ieca i.urea_2_ca35##c.LED_sqrt, efron

margins, at(LED_sqrt=(0(3)21) (asobserved) _all) over(urea_2_ca35) post

---------------------------------------------------------------------------------
                |            Delta-method
                |     Margin   Std. Err.      z    P>|z|     [95%
Conf. Interval]
----------------+----------------------------------------------------------------
_at#urea_2_ca35 |
           1 1  |   1.446149    .555207     2.60   0.009     .3579634
  2.534335
           1 2  |   1.133821   .6742893     1.68   0.093    -.1877618
  2.455403
more output not shown............

So, my question is why I am getting in the adjusted model a hazard
ratio of 1.4 instead of 1 for 1 1?

I appreciate your comments on this aparent contradiction.

Best wishes,
Eduardo




On Tue, Oct 18, 2011 at 4:11 PM, Richard Williams
<richardwilliams.ndu@gmail.com> wrote:
> At 11:53 AM 10/18/2011, Eduardo Nunez wrote:
>>
>> I appreciate any help in understanding this wonderful addition
>> (margins & marginsplot) to Stata 11 & 12.
>
> I don't know if it is a wonderful explanation, but here are the slides for a
> talk I recently gave, which in turn is an expanded version of a talk I gave
> at the Stata Conference this summer.
>
> http://www.nd.edu/~rwilliam/xsoc73994/Margins01.pdf
>
>
> -------------------------------------------
> Richard Williams, Notre Dame Dept of Sociology
> OFFICE: (574)631-6668, (574)631-6463
> HOME:   (574)289-5227
> EMAIL:  Richard.A.Williams.5@ND.Edu
> WWW:    http://www.nd.edu/~rwilliam
>
> *
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>

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