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From |
Maarten buis <[email protected]> |

To |
[email protected] |

Subject |
Re: st: Interaction between continious and categorical variable in Cox model |

Date |
Wed, 26 May 2010 07:46:06 +0000 (GMT) |

--- On Wed, 26/5/10, Sanam P wrote: > I have included interaction between a categorical variable > with 4 levels with a continuous variable in a Cox regression > model using : > > xi:stcox ... i.categorical*continuous > > The model now include below variables involved in the > interaction, with their Hazards ratios and p-values. > > > Categorical-2 > Categorical-3 > Categorical-4 > Continuous > Categorical X continious~2 > Categorical X continious~3 > Categorical X continious~4 > > I think that HR of "categorical-2" means the risk in > second category of the categorical variable compared with > first category when the continious variable is equal to > zero. and the hazards related to the "continiuos' means the > slope of the continious variable at first level of the > categorical variable. > > I was wondering whether you could kindly guide me how to > calculate and interpret hazards for the last 3 terms > involved in the interaction term. Consider the example below: *----------- begin example ------------ sysuse cancer, clear stset studytime, failure(died) replace age = age - 50 xi: stcox i.drug*age *------------ end example -------------- (For more on examples I sent to the Statalist see: http://www.maartenbuis.nl/example_faq ) The hazard ratios for _Idrug_2 says that the hazard of death for someone taking drug 2 is (.1308-1)*100% = -87% smaller than that hazard of someone taking drug 1 (the placebo) given that they are 50 years old. The effect of age says that for someone who receives the placebo, the hazard increases by 11% for every year (s)he gets older. The effect of _IdruXage_2 says that the effect of receiving drug 2 increases by 4% for every year that the patient gets older. Alternatively, you can also say that the effect of age increases by 4% if the patient gets drug 2 rather than the placebo. Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * 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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