Survival distributions impact the power of randomized placebo-phase design and parallel groups randomized clinical trials

J Clin Epidemiol. 2011 Mar;64(3):286-92. doi: 10.1016/j.jclinepi.2010.02.021. Epub 2010 Oct 6.


Objectives: The study evaluated the power of the randomized placebo-phase design (RPPD)-a new design of randomized clinical trials (RCTs), compared with the traditional parallel groups design, assuming various response time distributions. In the RPPD, at some point, all subjects receive the experimental therapy, and the exposure to placebo is for only a short fixed period of time.

Study design and setting: For the study, an object-oriented simulation program was written in R. The power of the simulated trials was evaluated using six scenarios, where the treatment response times followed the exponential, Weibull, or lognormal distributions. The median response time was assumed to be 355 days for the placebo and 42 days for the experimental drug.

Results: Based on the simulation results, the sample size requirements to achieve the same level of power were different under different response time to treatment distributions. The scenario where the response times followed the exponential distribution had the highest sample size requirement. In most scenarios, the parallel groups RCT had higher power compared with the RPPD.

Conclusion: The sample size requirement varies depending on the underlying hazard distribution. The RPPD requires more subjects to achieve a similar power to the parallel groups design.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Computer Simulation / standards*
  • Humans
  • Placebos
  • Randomized Controlled Trials as Topic / standards*
  • Randomized Controlled Trials as Topic / statistics & numerical data
  • Rare Diseases / mortality*
  • Research Design / standards
  • Sample Size
  • Survival Analysis


  • Placebos