Which of the following is not a type of clinical trial design?

A sequential trial is a design often discussed in the context of statistical analysis, particularly in adaptive clinical trials where data is evaluated as it is collected to determine if the trial should continue or stop early. However, it is not classified as a standalone clinical trial design in the same way that other options are.

In contrast, a single-site trial is conducted at one location, which can simplify logistics and control variability. A crossover trial involves participants receiving multiple treatments in a specific order, allowing each participant to serve as their own control, ultimately enhancing the ability to draw comparisons. A multi-center trial involves multiple locations participating in the study, which helps in generalizing results across diverse populations and settings.

Thus, while sequential methods may play a role in analyzing clinical trial data, they do not constitute a recognized category of clinical trial design like the other three listed options.