[R] Why is transform="km" the default for cox.zph?

[Kevin E. Thorpe]

To enhance my understanding, and that of my students, I have a question
about cox.zph in the survival package.

If I have correctly gleaned the high-level point from the 1994
Biometrika paper of Grambsch and Therneau, it looks to me like
cox.zph provides a mechanism to test for a simple trend in plots
of a function of time, g(t) versus the scaled schoenfeld
residuals and it also provides some built-in ones and the capability
to provide your own.  It also appears to me that different forms look
at different departures from proportionality.

So, my question is what are the advantages and disadvantages of the
default transform="km" compared to say, identity or log?

Thank you.

Kevin

-- 
Kevin E. Thorpe
Biostatistician/Trialist, Knowledge Translation Program
Assistant Professor, Department of Public Health Sciences
Faculty of Medicine, University of Toronto
email: kevin.thorpe at utoronto.ca  Tel: 416.946.8081  Fax: 416.946.3297