Diana wants to see if heat causes happiness. She asks 100 participants to come to the laboratory, and as they walk in, she asks each person to choose a warm booth or a cool booth. On the basis of their choices, participants spend 20 minutes in one or the other booth before rating their levels of general happiness. What's wrong with Diana's experiment?

she did not randomly assign participants to the experimental and control groups

Explanation:

In the scenario being described, the thing that is mostly wrong with Diana's experiment is that she did not randomly assign participants to the experimental and control groups. By not assigning the individuals to random booths and creating a control group then the individuals all chose the booth that they thought they would enjoy the most. This, as well as not being able to compare the results of each group ultimately ruins the overall reason for the experiment.