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From |
Eduardo Nunez <enunezb@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
st: base category in margins output |

Date |
Tue, 18 Oct 2011 12:53:29 -0400 |

Dear Statalisters, I have an interaction between a 4-level variable (1 to 4) and a continuous one (range between 0 to 21). Running a univariate Cox regression and margins afterward, I got defined the base category for the reported hazard ratios listed in the margins output (HR=1). However, when I run the same interaction, but, this time adjusted for other covariates, the margins output doesn't show the same HR (=1) even though is the base category. This is the example: Four-level variable (composed by grouping 2 binary variables) urea_2_ca35 | urea_2 ca35 ------------+-------------------- 1 | 1 0 2 | 0 0 3 | 0 1 4 | 1 1 ------------+-------------------- LED_sqrt=continuous variable with range from 0 to 21 Univariate Cox model (with level "1" as base category for urea_2_ca35 and "0" for LED_sqrt) stcox i.urea_2_ca35##c.LED_sqrt, efron Then I run margins as shown: 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............ I did show me (as expected) that all estimates (in hazard ratios scale) are using the category "11" as base category. So far so good. However, things get confused when I run the same interaction in a multivariable Cox. 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 Then I run the same margins as before: margins, at(LED_sqrt=(0(3)21) (asobserved) _all) over(urea_2_ca35) post And I got a hazard ratio for the category "11" of 1.446149 --------------------------------------------------------------------------------- | 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............ Can anyone explain me the meaning of this HR? It is not suppose to be 1 (base category for both interacting variables)? I appreciate any help in understanding this wonderful addition (margins & marginsplot) to Stata 11 & 12. I am using Stata 12 updated 13oct2011 Best regards, Eduardo * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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