Simultaneous sequential monitoring of efficacy and safety led to masking of effects

Rik van Eekelen, Esther de Hoop, Ingeborg van der Tweel

Research output: Contribution to journalArticleAcademicpeer-review


OBJECTIVE: Usually, sequential designs for clinical trials are applied on the primary (=efficacy) outcome. In practice, other outcomes (e.g., safety) will also be monitored and influence the decision whether to stop a trial early. Implications of simultaneous monitoring on trial decision making are yet unclear. This study examines what happens to the type I error, power, and required sample sizes when one efficacy outcome and one correlated safety outcome are monitored simultaneously using sequential designs.

STUDY DESIGN AND SETTING: We conducted a simulation study in the framework of a two-arm parallel clinical trial. Interim analyses on two outcomes were performed independently and simultaneously on the same data sets using four sequential monitoring designs, including O'Brien-Fleming and Triangular Test boundaries. Simulations differed in values for correlations and true effect sizes.

RESULTS: When an effect was present in both outcomes, competition was introduced, which decreased power (e.g., from 80% to 60%). Futility boundaries for the efficacy outcome reduced overall type I errors as well as power for the safety outcome.

CONCLUSION: Monitoring two correlated outcomes, given that both are essential for early trial termination, leads to masking of true effects. Careful consideration of scenarios must be taken into account when designing sequential trials. Simulation results can help guide trial design.

Original languageEnglish
Pages (from-to)155-65
Number of pages11
JournalJournal of Clinical Epidemiology
Publication statusPublished - Aug 2016


  • Biomedical Research/methods
  • Clinical Trials as Topic
  • Decision Making
  • Humans
  • Models, Statistical
  • Patient Safety
  • Research Design
  • Treatment Outcome

Cite this