304 PART 6 Analyzing Survival Data

You can describe these two situations in one general way. You know that every

participant in the study either died on a certain date (in which case they have the

event), or was alive up to some last-seen date when they stopped being observed,

in which case they are censored.

Figure 21-1 shows the results of a small study of survival in cancer patients after

a surgical procedure to remove a tumor. Ten patients were recruited to participate

in the study and were enrolled at the time of their surgery. The recruitment period

went from Jan. 1, 2010, to the end of Dec. 31, 2011 (meaning a two-year enrollment

period). All participants were then followed until they died, or until the conclusion

of the study, on Dec. 31, 2016, which added five years of additional observation

time after the last enrollment. Each participant has a horizontal timeline that

starts on the date of surgery and ends with either the date of death or the

censoring date.

In Figure 21-1, observe that each line ends with a code, and there’s a legend at the

bottom. Six of the ten participants (#’s 1, 2, 4, 6, 9, and 10, labeled X) died during

the course of the follow-up study. Two participants (#5 and #7, labeled L) were

LFU at some point during the study, and two participants (#3 and #8, labeled E)

were still alive at the end of the study. So this study has four participants — the

Ls and the Es — with censored survival times.

FIGURE 21-1:

Survival of ten

study participants

following surgery

for cancer.

© John Wiley & Sons, Inc.