Final answer:
In phase 3 studies, a control arm does exist, typically using a placebo or standard treatment. Designs can include randomized-sequenced double blind, placebo-controlled crossover studies or randomized, open-label, crossover studies.
Step-by-step explanation:
In phase 3 studies, which are a crucial stage in the development of new drugs and treatments, there is indeed usually a control arm. This control can either be a placebo or a standard treatment, and its purpose is to provide a benchmark against which the new intervention can be compared. Phase 3 studies are often randomized, and they can vary in their design. Some follow a randomized-sequenced double blind, placebo-controlled crossover design, where participants receive both the placebo and the active treatment in succession, blind to the order. Others might be randomized, open-label, crossover studies, where the participants know which treatment they are receiving and may receive both treatments in sequence for comparison.