You may not be able to observe all the participants in the data until they have the event.

Because of time constraints, at some point, you have to end the study and analyze your data. If your

endpoint is death, hopefully at the end of your study, some of the participants are still alive! At that

point, you would not know how much longer these participants will ultimately live. You only know

that they were still alive up to the last date they were measured in the study as part of data

collection, or the last date study staff communicated with them in some way (such as through a

follow-up phone call). This date is called the date of last contact or the last-seen date, and would

be the date that these participants would be censored in your data.

You may lose track of some participants during the study. Participants who enroll in a study

may be lost to follow-up (LFU), meaning that it is no longer possible for study staff to locate them

and continue to collect data for the study. These participants are also censored at their date of last

contact, but in the case of LFU, this date is typically well before the observation period ends.

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.

© John Wiley & Sons, Inc.

FIGURE 21-1: Survival of ten study participants following surgery for cancer.