Sample Size -- Survival Analysis

Instructions: Enter parameters in the Red cells. Answer will appear in the Blue cells.

  • Q1 = proportion of subjects in Group 1 (exposed)
  • Q0 = 1 - Q1 = proportion of subjects in Group 0 (unexposed)
  • RH = Relative Hazard Group 1/Group 0

Alpha (Two-tailed)BetaQ1Q0RH

Instructions: This tells you how many events you need, but it doesn't tell you how many subjects you need to start out with in each group. For that you need to specify the "Baseline Event Rate" (events per unit time) in Group 0, the "Censoring Rate" (# censored per unit time) in Group 0 (assumed to be the same in Group 1), and average length of follow-up. If you don't know the Baseline Event Rate but do know the median survival time in Group 0, see the * below.

*Median Survival
Time in Group 0
Baseline Event Rate1.980

Baseline Event Rate*
events/unit time
in Group 0
1.980Median survival
time in Group 0
0.233events/unit time
in Group 1
2.971Median survival time
in Group 1

Censoring Rate
censored/unit time
in Group 0
0.300censored/unit time
in Group 1

Planned Average Length of Follow-Up
time units

Group 1Group 0
Cumul. Event Rate0.2870.3920.360

Software utilities by Michael Kohn ( and Mike Jarrett (

Disclaimer: Software is provided on an as-is basis and no warranty is made of any kind. Calculations are believed accurate but should be verified by the user