CHAPTER 22 Comparing Survival Times 317

Chapter 22

Comparing Survival

Times

T

he life table and Kaplan-Meier survival curves described in Chapter 21 are

ideal for summarizing and describing the time to the first or only occur-

rence of a particular event based on times observed in a sample of individu-

als. They correctly incorporate data that reflect when an individual is observed

during the study but does not experience the event, which is called censored data.

Animal and human studies involving endpoints that occur on a short time-scale,

like measurements taking during an experimental surgical procedure, may yield

totally uncensored data. However, the more common situation is that during the

observation period of studies, not all individuals experience the event, so you usu-

ally have censored data on your hands.

In biological research and especially in clinical trials (discussed in Chapter 5), you

often want to compare survival times between two or more groups of individuals.

In humans, this may have to do with survival after cancer surgery. In animals, it

may have to do with testing the toxicity of a potential therapeutic. This chapter

describes an important method for comparing survival curves between two groups

called the log-rank test, and explains how to calculate the sample size you need to

have sufficient statistical power for this test (see Chapter 3). The log-rank test can

be extended to handle three or more groups, but this discussion is beyond the

scope of this book.

IN THIS CHAPTER»

» ^{Using the log-rank test to compare}

two groups»

» ^{Thinking about more complicated}

ways to compare the survival

experience»

» ^{Calculating the necessary sample size}

to compare survival times