[R] coxph and completely missing strata/subsetting

[Erik Iverson]

> I'm doing some coxph() analyses with a large and complex dataset. The
> data was collected in different centers, so I am using
> strata(centers) to stratify the analysis.
> 
> My main issue is, not all centers collected all the variables, so for
> a model such as:
> 
> coxph(Surv(days, cancer) ~ varA + sex + strata(centers), data)
> 
> I might have 1 or more centers that have NA for varA (in practice,
> all the individuals monitored at those centers come without varA).
> 
> coxph() obviously warns me that a number of individuals have been
> excluded -- would that be equivalent to doing the analysis on a
> subset of the data or not?

It seems so by definition to me.  I am confused as to what else would be 
happening.  You can always create a smaller, easier to understand 
dataset and try things out for yourself.  Perhaps I do not understand 
your question though.