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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