# Calculate number of events in a clinical trial design

Suppose I have 1000 patients enrolled in a trial (assuming 1:1 randomization to Treatment and Control arms), and my objective is to estimate the number of events (e.g., overall survival) needed to have a test of power at least 0.8 under the assumption of alpha=0.05. I wonder if there are formulas there to use, and/or R package that performs such power/sample size analyses. Thank you!

• You also need to take the estimated effect of the treatment into consideration.
– JonB
Sep 20 '15 at 19:46
• This approach is a bit backward from standard practice. Usually the size of the study is the result of power calculations based on event probabilities, estimated treatment effects, and the participant accrual pattern. Here one is starting already with a sample size of 1000. What is the purpose of the proposed calculation, if the sample size is already known?
– EdM
Sep 20 '15 at 23:37

• The R Hmisc package's cpower and spower functions help a bit. Sep 23 '15 at 11:57