R help - Apr 2014 - Time interactions for coxph object

Hi All,

I am working in the 'survival' library and need to know how to code a
variable time interaction for left truncated data (not all subjects
followed from the start of the study). I therefor add in the entry time and
exit time variables into the formula.

Here is my basic formula

    CSHR.shore.fly <- coxph(Surv(entry, exit, to == 1) ~ shore.cat, data
      glba.mod)

My variable shore.cat is violating the proportional hazards assumption so I
am trying to add in an interaction with time. Do I interact exit? entry? or
the range of the two?

Thanks, Tim

-- 
Tim Marcella
508.498.2989
timmarcella@gmail.com

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On Apr 7, 2014, at 9:36 AM, Tim Marcella wrote:
> Hi All,
>
> I am working in the 'survival' library and need to know how to code
a
> variable time interaction for left truncated data (not all subjects
> followed from the start of the study). I therefor add in the entry  
> time and
> exit time variables into the formula.
>
> Here is my basic formula
>
>    CSHR.shore.fly <- coxph(Surv(entry, exit, to == 1) ~ shore.cat,  
> data
>      glba.mod)
>
> My variable shore.cat is violating the proportional hazards  
> assumption so I
> am trying to add in an interaction with time. Do I interact exit?  
> entry? or
> the range of the two?
If you believe there is a trend in risk then there is the 'tt'  
argument which takes a function.

-- 
David Winsemius, MD
Alameda, CA, USA