What type of study design allows the investigator to examine the effects of multiple treatments simultaneously?

The correct answer is factorial study design. In a factorial study, researchers can investigate the effects of multiple treatments or interventions simultaneously by incorporating multiple factors, with each factor having different levels. This allows for an efficient examination of the interactions between the treatments. For example, if a study is assessing two drugs at two different doses, a factorial design enables evaluation of all combinations of those drugs and doses, providing insights into how they may interact with each other.

In contrast, crossover designs focus on the same participants receiving different treatments in a sequential manner, which is excellent for understanding within-subject effects but does not simultaneously analyze multiple treatments. Cross-sectional studies assess data at a single point in time but do not involve treatment application. Simple designs typically refer to basic comparative studies without the sophistication of handling multiple variables or treatments simultaneously, limiting their capacity to provide insights on interactions.