This manuscript summarizes and discusses fundamental issues in clinical trial design. A
clear understanding of the research question is a most important first step in designing a
clinical trial. Minimizing variation in trial design will help to elucidate treatment effects.
Randomization helps to eliminate bias associated with treatment selection. Stratified
randomization can be used to help ensure that treatment groups are balanced with respect to
potentially confounding variables. Blinding participants and trial investigators helps to
prevent and reduce bias. Placebos are utilized so that blinding can be accomplished. Control
groups help to discriminate between intervention effects and natural history. There are three
primary types of control groups: (1) historical controls, (2) placebo/sham controls, and (3)
active controls. The selection of a control group depends on the research question, ethical
constraints, the feasibility of blinding, the availability of quality data, and the ability to
recruit participants. The selection of entry criteria is guided by the desire to generalize the
results, concerns for participant safety, and minimizing bias associated with confounding
conditions. Endpoints are selected to address the objectives of the trial and should be
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NIH-PA Author Manuscript NIH-PA Author Manuscript NIH-PA Author Manuscript
clinically relevant, interpretable, sensitive to the effects of an intervention, practical and
affordable to obtain, and measured in an unbiased manner. Composite endpoints combine a
number of component endpoints into a single measure. Surrogate endpoints are measures
that are predictive of a clinical event but take a shorter time to observe than the clinical
endpoint of interest. Interim analyses should be considered for larger trials of long duration
or trials of serious disease or trials that evaluate potentially harmful interventions. Sample
size should be considered carefully so as not to be wasteful of resources and to ensure that a
trial reaches conclusive results.
There are many issues to consider during the design of a clinical trial. Researchers should
understand these issues when designing clinical trials