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

[Geraldine Henningsen]

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