[R] Problem plotting curve on survival curve (something silly?)

From: Calum <stats_at_wittongilbert.free-online.co.uk>
Date: Sun, 02 Mar 2008 22:14:57 +0000


OK this is bound to be something silly as I'm completely new to R - having started using it yesterday. However I am already warming to its lack of 'proper' GUI... I like being able to rerun a command by editing one parameter easily... try and do that in a Excel Chart Wizzard!

I eventually want to use it to analyse some chemotherapy response / survival data. That data will not be mature and so my interest is in using it to deliver predictions of what mature data may produce by using Weibull Regression

However for now I've borrowed a dataset from the BMJ (CSV tabs seperated version here:
http://www.wittongilbert.free-online.co.uk/HepatitisData.csv ) and am trying to plot some curves.

I've succeeded in plotting a Kaplan-Meier Survival Curve by using the following:

> my_var.bygroup <- survfit(Surv (Survival, Censored==0) ~
TreatmentGroup, data=TestData)
> plot(my_var.bygroup, conf.int=FALSE, col=c("black","grey"), lty=1:2,
legend.text=c("Placebo","Prednisolone"), main="dKaplan-Meier Survival Curve", xlab="Time (months)", ylab="Probability of Survival")

That produces this: http://www.wittongilbert.free-online.co.uk/RGraph.jpg

All of that is very nice so far. The I followed bits and pieces of other peoples posts in the past to plot on a weibull regression...

> my_curve.Plac <- survreg( Surv(Survival, Censored==0)~
TreatmentGroup, subset=TreatmentGroup=="Placebo", data=TestData, dist='weibull')
> curve(pweibull(x, scale=exp(coef(my_curve.Plac)),
shape=1/my_curve.Plac$scale, lower.tail=FALSE),from=0, col="black", to=max(TestData$Survival), add=TRUE)
> my_curve.Pred <- survreg( Surv(Survival, Censored==0)~
TreatmentGroup, subset=TreatmentGroup=="Prednisolone", data=TestData, dist='weibull')
> curve(pweibull(x, scale=exp(coef(my_curve.Pred)),
shape=1/my_curve.Pred$scale, lower.tail=FALSE),from=0, col="grey", to=max(TestData$Survival), add=TRUE)

This is where I have my problem - and I know its going to be something really silly! Look at my output:
http://www.wittongilbert.free-online.co.uk/RGraph2.jpg the trend line is zig-zagging between y=0 and the value it should be. If you draw a line along the tips of the zig-zags I guess thats a reasonable trendline...

Anyone able to help me on this?

Also is it possible to get an R-squared type value for the fit of this curve from someplace?

Finally (three questions in one!) the first two censored data points (1 in each group) are actually lost to follow-ups. Should they be marked differently from censored?

Sorry I know thats a mamoth post - and have a horrible feeling someone's going to say 'You are trying to do something thats statistically wrong!' - feel free!

Many thanks



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