Hi Enrico, Not sure how SAS builds the CI but I can look into it. The SAS documentation does have a section on computational formulas at:
http://support.sas.com/documentation/cdl/en/statug/63033/HTML/default/viewer.htm#statug_lifetest_a0000000259.htm Although I can't provide my dataset, I can provide the data and code below. This is the R-equivalent of an analysis from "Common Statistical Methods for Clinical Research with SAS Examples." R produces the follwoing output: > print(surv.by.vac) Call: survfit(formula = Surv(WKS, CENS == 0) ~ VAC, data = hsv) records n.max n.start events median 0.95LCL 0.95UCL VAC=GD2 25 25 25 14 35 15 NA VAC=PBO 23 23 23 17 15 12 35 SAS has the same 95% CI for VAC=GD2 but has a 95% CI of [10, 27] for VAC=PBO. This is just like in the analysis I'm doing currently. Thanks, Paul ####################################### #### Chapter 21: The Log-Rank Test #### ####################################### ##################################################### #### Example 21.1: HSV2 Vaccine with gD2 Vaccine #### ##################################################### connection <- textConnection(" GD2 1 8 12 GD2 3 -12 10 GD2 6 -52 7 GD2 7 28 10 GD2 8 44 6 GD2 10 14 8 GD2 12 3 8 GD2 14 -52 9 GD2 15 35 11 GD2 18 6 13 GD2 20 12 7 GD2 23 -7 13 GD2 24 -52 9 GD2 26 -52 12 GD2 28 36 13 GD2 31 -52 8 GD2 33 9 10 GD2 34 -11 16 GD2 36 -52 6 GD2 39 15 14 GD2 40 13 13 GD2 42 21 13 GD2 44 -24 16 GD2 46 -52 13 GD2 48 28 9 PBO 2 15 9 PBO 4 -44 10 PBO 5 -2 12 PBO 9 8 7 PBO 11 12 7 PBO 13 -52 7 PBO 16 21 7 PBO 17 19 11 PBO 19 6 16 PBO 21 10 16 PBO 22 -15 6 PBO 25 4 15 PBO 27 -9 9 PBO 29 27 10 PBO 30 1 17 PBO 32 12 8 PBO 35 20 8 PBO 37 -32 8 PBO 38 15 8 PBO 41 5 14 PBO 43 35 13 PBO 45 28 9 PBO 47 6 15 ") hsv <- data.frame(scan(connection, list(VAC="", PAT=0, WKS=0, X=0))) hsv <- transform(hsv, CENS = ifelse(WKS < 1, 1, 0), WKS = abs(WKS), TRT = ifelse(VAC=="GD2", 1, 0)) library("survival") surv.by.vac <- survfit(Surv(WKS,CENS==0)~VAC, data=hsv) plot(surv.by.vac, main = "The Log-Rank Test \n Example 21.1: HSV-Episodes with gD2 Vaccine", ylab = "Survival Distribution Function", xlab = "Survival Time in Weeks", lty = c(1,2)) legend(0.75,0.19, legend = c("gD2","PBO"), lty = c(1,2), title = "Treatment") summary(surv.by.vac) print(surv.by.vac) ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
