Which term best describes a study in which subjects receive both a treatment and a placebo at different intervals?

A crossover study is one in which participants receive both the treatment and a placebo at different intervals, allowing each subject to serve as their own control. This design has several advantages, as it reduces variability due to individual differences and enhances the statistical power of the study by allowing researchers to compare the effects of the treatment and placebo directly within the same individuals.

In a crossover study, each participant is assigned to different sequences of treatment, ensuring that all subjects eventually experience both conditions. This design is particularly useful in clinical research where the same individuals can experience both interventions, enabling a more reliable determination of the treatment's effects compared to a placebo.

The other terms do not fit this description. A cross-sectional study focuses on data collected at a single point in time, a double-blind study ensures that neither the participants nor the researchers know who is receiving the treatment or the placebo, and a longitudinal study follows subjects over an extended period to observe long-term effects or trends.