\name{survobrien}
\alias{survobrien}
\title{
O'Brien's Test for Association of a Single Variable with Survival
}
\description{
Peter O'Brien's test for association of a single variable with survival
This test is proposed in Biometrics, June 1978.
}
\usage{
survobrien(formula, data)
}
\arguments{
\item{formula}{
a valid formula for a cox model, without time dependent covariates.
}
\item{data}{
a data frame.
}}
\value{
a new data frame.  The original time and status variables are removed,
and have been replaced with \code{start}, \code{stop}, and \code{event}.  If a
predictor variable is a factor or is protected with \code{I()}, it is
retained as is.  Other predictor variables have been replaced with
time-dependent logit scores.


Because of the time dependent variables, the new data frame will have many
more rows that the original data, approximately \#rows * \#deaths /2.
}
\section{METHOD}{
A time-dependent cox model can now be fit to the new data.
The univariate statistic, as originally proposed, is equivalent to
single variable score tests from the time-dependent model.
This equivalence is the rationale for using the time dependent model as a
multivariate extension of the original paper.


In O'Brien's method, the x variables are re-ranked at each death time.  A
simpler method, proposed by Prentice, ranks the data only once at the
start. The results are usually similar.
}
\references{
O'Brien, Peter, "A Nonparametric Test for Association with Censored Data",
\emph{Biometrics} 34: 243-250, 1978.
}
\seealso{
\code{\link{survdiff}}
}
\keyword{survival}
\examples{
xx <- survobrien(Surv(futime, fustat) ~ age + factor(rx) + I(ecog.ps),
			       data=ovarian)
coxph(Surv(start, stop, event) ~ age, data=xx)
coxph(Surv(start, stop, event) ~ age + rx + ecog.ps, data=xx)
}
% Converted by Sd2Rd version 0.3-2.
