Re: [R] cpower and censoring

From: Frank E Harrell Jr <>
Date: Thu, 08 May 2008 08:02:19 -0500

Daniel Brewer wrote:
> I would like to do some power estimations for a log-rank two sample test
> and cpower seems to fit the bill. I am getting confused though by the
> man page and what the arguments actually mean. I am also not sure
> whether cpower takes into account censoring or not.
> Could anyone provide a simple example of how I would get the power for a
> set control/non-control clinical trial where censoring occurs at an
> estimated rate with an estimated drop out rate.
> Quite confused about this.

cpower handles censoring by specification of an accrual and follow-up periods and the event probability by reference time tref. I believe the censoring distribution is assumed to be uniform, i.e., that accrual occurs at a uniform rate.

To have more control over the situation, the simulation-based spower function is one to look into.

Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University

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