[R] Mean survival times

[Christoph Scherber]

Dear list,

I have data on insect survival in different cages; these have the 
following structure:

  deathtime status id cage    S      F     G   L     S
        1.5      1  1 C1      8      2     1   1     1
        1.5      1  2 C1      8      2     1   1     1
       11.5      1  3 C1      8      2     1   1     1
       11.5      1  4 C1      8      2     1   1     1

There are 81 cages and each 20 individuals whose survival was followed 
over time. The columns S,F,G,L and S are experimentally manipulated 
factors thought to have an influence on survival.

Using survfit(Surv(deathtime,status)~cage) gives me the survivorship 
curves for every cage. But what I´d like to have is a mean survivorship 
value for every cage.

Obviously, using tapply (deathtime,cage,mean) gives me mean values, but 
I´d like to have a better estimate of this using a proper statistical 
model. I´ve tried a glm with poisson errors (as suggested in Crawley´s 
book, page 628), but the back-transformed estimates (using status as the 
response variable and deathtime as an offset) were totally unrealistic.

As I´m new to survival analysis, it would be great if anyone could give 
me some hints on what method would be best.

Thanks a lot!
Christoph