In which study design are participants unaware of whether they are receiving the active treatment or a placebo?

The design where participants do not know whether they are receiving the active treatment or a placebo is a randomized control trial (RCT). This blinding is a critical component of RCTs because it helps to minimize bias in the results. By ensuring that neither the participants nor the researchers know who is receiving the treatment and who is receiving the placebo, RCTs enhance the reliability of the findings. This helps to ensure that any differences observed in outcomes can be attributed to the treatment itself rather than to participants' expectations or biases regarding their treatment status.

In contrast, cross-sectional studies involve collecting data at a single point in time and do not typically include interventions or treatments. Observational studies focus on observing participants without manipulating the study environment or providing treatments. Longitudinal studies track participants over time, often collecting data at various points, but they do not inherently involve randomization or blinding with respect to treatment versus placebo. Therefore, RCTs are distinct in their design and methodology regarding participant awareness of treatment versus placebo.