[R] Interval censored Data in survreg() with zero values!

[Achim Zeileis]

On Tue, 23 Dec 2008, Geraldine Henningsen wrote:

> Hello,
>
> I have interval censored data, censored between (0, 100). I used the
> tobit function in the AER package which in turn backs on survreg.
> Actually I'm struggling with the distribution. Data is asymmetrically
> distributed, so first choice would be a Weibull distribution.
> Unfortunately  the Weibull doesn't allow for zero values in time data,
> as it requires x > 0. So I tried the exponential distribution that
> allows x to be >= 0 and the log-normal that sets x <= 0 to 0. Still I
> get the same error:
>
> "     Fehler in survreg(formula = Surv(ifelse(A16_1_1 >= 100, 100,
> ifelse(A16_1_1 <=  :
>  Invalid survival times for this distribution "
>
> The only distributions that seem to work are gaussian and logistic, but
> they don't really fit the data.
> I searched for this problem in the archive and found a suggestion by
> Terry Therneau to set all 0  to NA, applying Weibull afterwards.  But
> this solution is not very satisfying as it eliminates the left censored
> data from the dataset.
>
> So I have three questions:
>
> 1. Does anybody know why the lognormal and exponential distribution
> don't work in survreg?

For these distributions, observations left-censored at zero are rather 
unlikely to occur: pexp(0) = plnorm(0) = 0.

> 2.  What else could I do to find a distribution that fits the data well?
>
> 3. What about the non-parametric approach in survfit(), could that be a
> solution?

Both probably depend on the questions you want to ask about your data. For 
the tools implemented in "survival", the "Modeling Survival Data" book by 
Therneau and Grambsch is the natural reference.

hth,
Z

> I hope my question aren't too stupid, as I'm not a big statistician.
>
> Regards,
>
> Geraldine
>
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