For Questions , Use the Following Narrative

For Questions , use the following narrative
Narrative: Marijuana and brain
Researchers at the University of Iowa College of Medicine reported that a test showed those who smoked seven or more marijuana joints per week had lower math scores than non-marijuana users.A related headline says "New study confirms too much pot impairs brain".
-{Narrative: Marijuana and brain} Explain why these results must have been based on an observational study, and not an experiment.