A "single-blind" study is a type of research design where the participants do not know whether they are receiving the treatment or a placebo. This helps to prevent bias in their responses or behaviors, as they are unaware of which group they belong to. However, the researchers conducting the study do know which participants are receiving the treatment. This knowledge allows them to monitor the effects and gather data effectively, but it can also introduce potential bias in how they interact with participants or interpret results. Single-blind studies are commonly used in clinical trials involving medications or therapies.