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From |
"Weichle, Thomas" <Thomas.Weichle@va.gov> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
st: margins command after stcox to predict adjusted survival function |

Date |
Wed, 22 Jun 2011 10:18:10 -0500 |

Hi Statalisters, I'd like to use the -margins- command for predicting the adjusted survival function. Using -margins- as post estimation to -stcox- will produce the relative hazard as the default (predict(hr)). However, after running -stcox- and predicting the baseline survival function, it appears as if I can use -margins- with the expression() option to produce the adjusted survival since S(t) = [S0(t)]^exp(xb). I would like to know if this conceptually makes sense. stcox c_dx_age black i.married i.chrlson_grp_6mp_dx surgery_tvc i.va_outpt_dist_tertile /// outpt_event_q4 i.dual2_cc_claims_2 hs_edu10 ib4.region i.grade2 if stage_ajcc5 == "I", nolog predict double S0, basesurv margins i.dual2_cc_claims_2, expression(S0^exp(predict(xb))) post ------------------------------------------------------------------------ ------ | Delta-method | Margin Std. Err. z P>|z| [95% Conf. Interval] -------------+---------------------------------------------------------- ------ dual2_cc_c~2 | 0 | .8873086 .032696 27.14 0.000 .8232256 .9513915 1 | .8370149 .0467662 17.90 0.000 .7453547 .928675 ------------------------------------------------------------------------ ------ These predicted margins for the 2 levels of dual2 appear to be the adjusted average survival probabilities. On a side note, suppose I run the same -stcox- model and then run -stcurve, survival- for each level of dual2. If I take the average of the survival probabilities for each of 2 levels of dual2 produced from -stcurve-, this is identical to what -margins- would have produced if I used the atmeans options. stcurve, survival at1(dual2_cc_claims_2=0) at2(dual2_cc_claims_2=1) margins i.dual2_cc_claims_2, expression(S0^exp(predict(xb))) atmeans post This makes sense because the -stcurve- command evaluates the function by setting each covariate to its mean value. And this is exactly what is being done when I specify the atmeans option for the -margins- command. Tom Weichle Math Statistician Center for Management of Complex Chronic Care (CMC3) Hines VA Hospital, Bldg 1, C202 708-202-8387 ext. 24261 Thomas.Weichle@va.gov * * 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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