ins <- read.table("~/public_html/Insul.csv",sep=",",header=T)
 attach(ins)  ; names(ins)
 kVolt <- as.factor(kV) # this is necesary to get separate KM curves.

 insul.km <- survfit(Surv(tc2,cens2)~ kVolt,conf.type="log-log")
   
 survfit(insul.km) 
   # find out how to use different line types, and try adding legends.

 survdiff(Surv(tc2,cens2)~ kVolt)   
   # what is the pattern of observed versus expected?
   # what does this suggest about the power of this test?

 fail <- rep(1,length( t)) # a failure indicator for all times. 
   # for comparision: the full data. 
 plot(survfit(Surv(t/60,fail)~ kVolt,conf.type="log-log"),
    xlab="Hours", main="Breakdown  by voltage level")
   # notice how functions can be nested. 
 survdiff(Surv(t,fail)~ kVolt)