Clinical Trials

3. Classification

Number of centers

Clinical trials can also be classified as single-center or multicenter studies according to the number of sites involved. While single-site studies are mainly used for Phase I and II studies, multicenter studies can be carried out at any stage of clinical development.

Multicenter studies are necessary for two major reasons (Truesdale et al., 2006; Matthews, 2000):

• To evaluate a new medication or procedure more efficiently in terms of accruing sufficient subjects over a shorter period of time; and
• To provide a better basis for the subsequent generalization of the trial’s findings, i.e., the effects of the treatment are likely to be similar in a wider setting across centers not involved in the trial.

Other classifications

Trials can also be described as superiority studies, equivalence studies, or noninferiority studies in terms of what the study was designed to prove.

• A superiority study aims to show that a new drug is more effective than the comparative treatment (placebo or current best treatment) (Pocock, 1983; Chow et al., 2003). Most clinical trials belong to this category.
• On the other hand, an equivalence trial is designed to prove that two drugs have the same clinical benefit. Hence, the trial should demonstrate that the effect of the new drug differs from the effect of the current treatment by a margin that is clinically unimportant (Bakhai et al., 2006c; Wang et al., 2006a).
• A noninferiority trial aims to show that the effect of a new treatment cannot be said to be significantly weaker than that of the current treatment.

In the latter two trials the new treatment might still turn out to be more effective than the comparative treatment, but this is not the prior assumption of the trial (Miller et al., 2006).