[R] nesting in CoxPH with survival package

[Bert Gunter]

Katie:

You need to get local statistical help. What you are doing makes no
sense. See inline below.

-- Bert

On Sat, Jan 12, 2013 at 1:03 PM, David Winsemius <dwinsemius at comcast.net> wrote:
>
> On Jan 11, 2013, at 5:35 PM, Katie Anweiler wrote:
>
>> Hello all,
>> I am trying to understand how to specify nested factors when using
>> coxph(), and if it is appropriate to nest these factors in my
>> situation.
>> In the simplest form, I am testing two different temperatures, with
>> each temperature being performed twice in different experimental
>> periods (e.g. Temp5 performed in Period A and C, Temp4 performed in
>> Period B and D)

Period is confounded with temperature. That is the source of the
singularity. in the message received below.

You can estimate the C-A and the D-B differences.
As I said, get statistical help. These are not R questions.

-- Bert


>> I am trying to see if survival time is affected by the treatment
>> temperature.  To do this I am using temperature and experimental
>> period nested within temperature as factors.
>>
>>> LOEtempmod.5days=coxph(LOE.stable.5days~Temp+Temp/Period,data=goodstable)
>>
>> Warning message:
>> In coxph(LOE.stable.5days ~ Temp + Temp/Period,  :
>>  X matrix deemed to be singular; variable 2 5 6 7
>>
>> 1. Is this an appropriate way of nesting?
>
>
> Have you looked at the coxme package?
> http://cran.r-project.org/web/packages/coxme/index.html
>
>> 2. Can this error message be ignored?
>
>
> Sometimes R packages correctly drop variables that are exactly collinear:
> other times the correct solution is not clear. I would think the answer in
> this case would be "no", but do not have a lot to go on at this point.
>
> --
>
> David Winsemius, MD
> Alameda, CA, USA
>
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> and provide commented, minimal, self-contained, reproducible code.



-- 

Bert Gunter
Genentech Nonclinical Biostatistics

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