[R] coxph and completely missing strata/subsetting

[Federico Calboli]

Hi everyone,

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? 

I ask because I have many centers and many variables, and if the automatic exclusion of individuals missing the variable in analysis *is not* equivalent to subsetting I might have some serious work to do.

Best,

Federico

--
Federico C. F. Calboli
Department of Epidemiology and Biostatistics
Imperial College, St. Mary's Campus
Norfolk Place, London W2 1PG

Tel +44 (0)20 75941602   Fax +44 (0)20 75943193

f.calboli [.a.t] imperial.ac.uk
f.calboli [.a.t] gmail.com