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

[Don MacQueen]

Surv() allows left, right, or interval censoring.

Try left censoring instead of interval censoring. For the weibull or 
lognormal, think of your data as <=100 instead of [0,100].

-Don

At 8:08 PM +0100 12/23/08, 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?
>
>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?
>
>I hope my question aren't too stupid, as I'm not a big statistician.
>
>Regards,
>
>Geraldine
>
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-- 
--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA
925-423-1062