CHAPTER 22 Comparing Survival Times 325

University

Medical

Center

(https://biostat.app.vumc.org/wiki/Main/

PowerSampleSize).

After opening the PS program, choose the Survival tab, fill in the form, and click

Calculate. The median survival times for the two groups are labeled m1 and m2,

the accrual interval is labeled A, the post-accrual follow-up period is labeled F,

and the group allocation proportion is labeled m. Note that the time variables must

always be entered in the same units (days, in this example). You will also need to

enter your chosen α and power.

Here is an example. Suppose that you’re planning a study to compare an experi-

mental drug for keeping cancer remission to placebo in two equal-sized groups of

cancer patients whose cancer is in remission. You expect to observe participants

for a total of three years to see whether their cancer returns (which is the out-

come). From existing studies, you expect the median placebo time to be 20 months,

and you think the drug should extend this to 30 months. If it truly does extend

survival (time to remission) that much, you want to be able to detect this. You set

α = 0.05 and power at 80 percent so that you have an 80 percent chance of getting

a p value of less than 0.05 when you compare drug to placebo using the log-rank

test. If you fill in the PS form with these estimates and select Calculate, under

Sample Size the software will say you need 170 participants in each group (a total

of 340 participants).