Survival function S(t)

S(t) is the estimated proportion of people for whom the event has not occurred by time t. It starts at 1 (everyone) and drops only at events. “Survival” is the medical technical term and here simply means “no event yet” - for example still in the program. Each value comes with a standard error (Greenwood) and a 95% confidence interval.

Example: S(500) = .30, 95% CI [.23, .40]: after 500 days an estimated 30% have not yet experienced the event.

Caution: The interval is computed on the log scale (as in R survfit): it never falls below 0, and the upper limit is truncated at 1. With few people at risk it is very wide.