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
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