Flexible designs of clinical trials – a graphical representation in form of a chance tree

Background: Flexible design of clinical trials may allow tailoring ongoing investigations to better suit actual circumstances such as an unexpectedly low effect size without compromising error control properties. Available statistical methods are, however, complex. Method: A two-stage flexible design is represented in form of a simple chance tree. Critical limits accounting for one interim analysis are derived using only basic rules of probability calculus. Conclusion: By means of chance trees the principles of flexible trial design may be communicated more easily, thus hopefully adding to good design and conduct of clinical trials.