A researcher is studying whether individuals drinking caffeinated coffee in the morning report more alertness in the afternoon. To do this, the researcher invites 100 participants to the lab in the morning. She randomly assigns the participants into two equal groups. One group is given caffeinated coffee and the other is given decaf coffee. The participants are not told which type of coffee the received. Then, later that day participants self-report how alert they feel. Because the research knows which group received which type of coffee and the participants did not, this is what kind of study