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# Re: st: semiparametric mixture cure models

 From "E. Paul Wileyto" To statalist@hsphsun2.harvard.edu Subject Re: st: semiparametric mixture cure models Date Tue, 23 Feb 2010 10:34:33 -0500

If you are talking about a Cox-type regression on top of a cure mixture, then no. These have real identifiability problems, and identification usually involves making an assumption about where the floor is.
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There is strsnmix, Paul Lambert's parametric cure-mixture model, and it works well. I have some home-grown stuff, but it is still parametric. If you have trouble with identification using the parametric model, it may be because the floor is vague in your data. Parametric cure models fit well if you can actually see the floor in a Kaplan Meier plot. If you do not see the survivorship leveling off in the KM plot, the floor of the cure model doesn't have any way to ID the floor in your data.
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Have you thought of modeling with frailty? You may achieve the same kind of result with a slightly different set of assumptions.
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P

Sridhar Telidevara wrote:
```
```Are there any stata routines for estimating a semiparametric mixture cure
model with fixed covariates?  The data have 30,000  observations and the
```
duration of observation is in days ( 1 day to 1096 days).
```I have estimated several parametric mixture cure models and they did not
yield good results. Cure percentage was very low, the fit had long tails and
the median survival time is greater than 1096 days. Therefore, there exists
an identification problem.

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Further,
```1.       since the data is in days, is stset enough or episode splitting is
required?

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```2.       Convergence problems with the EM algorithm and the bootstrapping
procedures if the data is in days.

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```3.       If I have to group the data to estimate a semiparametric mixture
cure model what is the optimal way to group the data? 5 days, 10 days? 30
days?

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```Thank you for your help,

```
```Sridhar Telidevara

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```
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--
E. Paul Wileyto, Ph.D.
Assistant Professor of Biostatistics
Tobacco Use Research Center
School of Medicine, U. of Pennsylvania
3535 Market Street, Suite 4100

215-746-7147
Fax: 215-746-7140
```
epw@mail.med.upenn.edu
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```