Final answer:
Dr. Poldark is using a randomized controlled trial, where participants are assigned to either a control or experimental group to determine the effect of stress, allowing for causal inferences.
Step-by-step explanation:
Dr. Poldark is conducting a randomized controlled trial, which is a type of experimental study design. In this design, participants are randomly assigned to either a control group or an experimental group. The control group experiences a non-stressful condition while the experimental group faces a stressful condition. The random assignment is key for reducing selection bias and ensuring that the two groups are comparable at the start of the experiment. This allows Dr. Poldark to infer causality and attribute any differences in outcomes between the two groups to the stress manipulation.
Unlike cross-sectional studies, which measure variables at one point in time, or longitudinal studies, which follow the same group over a period of time, randomized controlled trials are controlled experiments that can establish cause-and-effect relationships between the independent (stress) and dependent variables.