Basic Statistics

206 INTRODUCTION TO SURVIVAL ANALYSIS

f( t) or proportion dead

0 tl 2 3

Time (years)

Figure 14.4 Cumulative death density function, F(t).

Time (years)

Figure 14.5 Survival function, S(t).

14.2.3 The Survival Function

Figure 14.5 displays the survival function. Note that this is simply Figure 14.4 flipped
over. The survival function S(t) starts at 1 at time 0 (all the patients are alive) and
decreases to 0 at time 3. Sometimes the results are multiplied by 100 and reported
in percent. We would say that 100% survived at time 0. Just as it was difficult to
decide on the shape of the death density function by looking at the cumulative death
distribution function, it is also difficult to use the survival function for this purpose.