Let's assume that we have a survfit object as follows.
fit = survfit(Surv(data$time_12m, data$status_12m) ~ data$group)
fit
Call: survfit(formula = Surv(data$time_12m, data$status_12m) ~ data$group)
n events median 0.95LCL 0.95UCL
data$group=HF 10000 3534 NA NA NA
data$group=IGT 70 20 NA NA NA
fit object does not show CI-s. How to calculate confidence intervals for the survival rates? Which R packages and code should be used?
The print result of survfit gives confidnce intervals by group for median survivla time. I'm guessing the NA's for the estimates of median times is occurring because your groups are not having enough events to actually get to a median survival. You should show the output of plot(fit) to see whether my guess is correct.
You might try to plot the KM curves, noting that the plot.survfit function does have a confidence interval option constructed around proportions:
plot(fit, conf.int=0.95, col=1:2)
Please read ?summary.survfit. It is the class of generic summary functions which are typically used by package authors to deliver the parameter estimates and confidence intervals. There you will see that it is not "rates" which are summarized by summary.survfit, but rather estimates of survival proportion. These proportions can either be medians (in which case the estimate is on the time scale) or they can be estimates at particular times (and in that instance the estimates are of proportions.)
If you actually do want rates then use a functions designed for that sort of model, perhaps using ?survreg. Compare what you get from using survreg versus survfit on the supplied dataset ovarian:
> reg.fit <- survreg( Surv(futime, fustat)~rx, data=ovarian)
> summary(reg.fit)
Call:
survreg(formula = Surv(futime, fustat) ~ rx, data = ovarian)
Value Std. Error z p
(Intercept) 6.265 0.778 8.05 8.3e-16
rx 0.559 0.529 1.06 0.29
Log(scale) -0.121 0.251 -0.48 0.63
Scale= 0.886
Weibull distribution
Loglik(model)= -97.4 Loglik(intercept only)= -98
Chisq= 1.18 on 1 degrees of freedom, p= 0.28
Number of Newton-Raphson Iterations: 5
n= 26
#-------------
> fit <- survfit( Surv(futime, fustat)~rx, data=ovarian)
> summary(fit)
Call: survfit(formula = Surv(futime, fustat) ~ rx, data = ovarian)
rx=1
time n.risk n.event survival std.err lower 95% CI upper 95% CI
59 13 1 0.923 0.0739 0.789 1.000
115 12 1 0.846 0.1001 0.671 1.000
156 11 1 0.769 0.1169 0.571 1.000
268 10 1 0.692 0.1280 0.482 0.995
329 9 1 0.615 0.1349 0.400 0.946
431 8 1 0.538 0.1383 0.326 0.891
638 5 1 0.431 0.1467 0.221 0.840
rx=2
time n.risk n.event survival std.err lower 95% CI upper 95% CI
353 13 1 0.923 0.0739 0.789 1.000
365 12 1 0.846 0.1001 0.671 1.000
464 9 1 0.752 0.1256 0.542 1.000
475 8 1 0.658 0.1407 0.433 1.000
563 7 1 0.564 0.1488 0.336 0.946
Might have been easier if I had used "exponential" instead of "weibull" as the distribution type. Exponential fits have a single parameter that is estimated and are more easily back-transformed to give estimates of rates.
Note: I answered an earlier question about survfit, although the request was for survival times rather than for rates. Extract survival probabilities in Survfit by groups
