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# Re: st: Cox survival-patients who die at entry

 From Steve Samuels To statalist@hsphsun2.harvard.edu Subject Re: st: Cox survival-patients who die at entry Date Tue, 23 Apr 2013 22:18:43 -0400

To expand on Adam's comment:

Estimated coefficients from -stcox- can be affected by the exact value
of the added fraction (e.g. 0.1  vs. 0.5) only if the predictors include
interactions with time. In -stcox-, these are set by the tvc() and
texp() options. In the absence of such interactions, times enter into
the partial likelihood equation only through their rank order.

On the other hand, the contributed (SSC) flexible regression commands -stpm-,
-stpm2- and -stgenreg- are parametric models in time. The estimated
coefficients from these commands can be affected by the exact value of
the added fraction even if the predictors include no interactions with
time.

Steve

On Apr 22, 2013, at 3:15 PM, Adam Olszewski wrote:

If the patient has survival time zero, then the patient contributes
zero (1 person * 0 days) to time at risk. Those patients will be
excluded from the survival analysis at the outset, when you -stset-
I agree with the comment to add a fraction of the day to the data,
although in some settings I found that the Cox regression coefficients
can change depending on how much you add (0.1 vs. 0.5 etc.) and how
many patients are affected.
AO

On Mon, Apr 22, 2013 at 1:37 PM, Steve Samuels <sjsamuels@gmail.com> wrote:
> I assume you have dates in days:
>
> . replace sdate = sdate + 0.5 if (died on day of enrollment)
> . stset stdate
>
> Use more exact fractions if you have them.
>
> In some circumstances, a lot of ties can distort the analysis. If you have frequent ties for  days of early deaths, then perhaps you should ascertain approximate times of death for all patients with early deaths, not just those who died on day 1.
>
> Steve
> sjsamuels@gmail.com
>
>
> On Apr 22, 2013, at 1:11 PM, Po-Lin Chan wrote:
>
> Hi Statalisters,
>
> I am running a Cox survival but have 5 patients who die on the day of
> enrolment (ie entry).
> Does Stata automatically account for the contribution of these 5 patients
> for the total patient time?
> Do I need to add additional time to their date of death in stata, and how
> do I do this?
>
> Thank you
>
> Polin Chan
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