[R] How to stop Kaplan-Meier curve at a time point

[Andrews, Chris]

That subset will give you right truncation, not right censoring.  See code below.  Use Thomas's solution.

Chris


library(survival)
set.seed(20131121)
ngroup <- 100
xxx <- rep(0:1, e=ngroup)
ttt <- rexp(length(xxx), rate=xxx+.5)
plot(survfit(Surv(ttt) ~ xxx))
survdiff(Surv(ttt) ~ xxx)

# impose earlier stop time?
tstop <- 2
lines(survfit(Surv(ttt) ~ xxx, subset=ttt<tstop), col=2)
survdiff(Surv(ttt) ~ xxx, subset=ttt<tstop)

# censor at earlier stop time
ddd <- ttt<tstop
ttt2<- pmin(ttt, tstop)

lines(survfit(Surv(ttt2, ddd) ~ xxx), col=3)
survdiff(Surv(ttt2, ddd) ~ xxx)
# green lines match black lines up to tstop




-----Original Message-----
From: David Winsemius [mailto:dwinsemius at comcast.net] 
Sent: Wednesday, November 20, 2013 5:49 PM
To: Dr.Vinay Pitchika
Cc: r-help at r-project.org
Subject: Re: [R] How to stop Kaplan-Meier curve at a time point


On Nov 20, 2013, at 12:01 PM, Dr.Vinay Pitchika wrote:

> Hello R users
> 
> I have a question with Kaplan-Meier Curve with respect to my research. We
> have done a retrospective study on fillings in the tooth and their survival
> in relation to the many influencing factors. We had a long follow-up time
> (upto 8yrs for some variables). However, we decided to stop the analysis at
> the 6year follow up time, so that we can have uniform follow-up time for
> all the variables.
> 
> I did play a bit with the formula and tried to stop the Kaplan-Meier curve
> at my desired time (2190days)or roughly 6 years. However, my question is I
> want to find the significance (log rank test) at this time point between
> the two curves; because I am not able to find a way to stop the survfit at
> this time point with my knowledge. Below is the code I used.
> 
> Gender2<-survfit(Surv(Survival_days, Outcome)~Gender)

I'm assuming that you have a dataframe with those variables and have attached it. If so, then:

dfrm <- detach(said_df)

# If not, then:

dfrm <- data.frame(Survival_days, Outcome, Gender)

Gender2<-survfit(Surv(Survival_days, Outcome)~Gender, 
                  data=dfrm, subset = Survival_days < 6*365.25 )

> 
> plot (Gender2, xmax=2190, mark.time=FALSE, col=c(1:2), xlab="Survival time
> (Days)", ylab="Survival probability", main="Gender") # mark.time=FALSE will
> remove the censoring lines in the graph. If censoring lines are needed,
> then remove the mark.time section in the formula.
> 
> legend("topright",c("Females", "Males"), col=(1:2), lwd=0.5)
> 
> Am sure, the code in the first line has to be modified. Please help me with
> this and I will be very thankful to you.
> 


David Winsemius
Alameda, CA, USA


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