A health story's headline reads, "A New Prescription For Depression: Join A Team And Get Sweaty"
Psychologists have known for a while that depression levels are linked to exercise. In fact, some comprehensive treatments for depression include an exercise component in addition to psychotherapy or medication.
This recent NPR story focused on which type of exercise that might work best:
..a recent study in the journal Lancet Psychiatry found that popular team sports may have a slight edge over the other forms of physical activity.
The researchers analyzed Centers for Disease Control and Prevention survey data from 1.2 million adults and found — across age, gender, education status and income — people who exercised reported fewer days of bad mental health than those who didn't. And those who played team sports reported the fewest.
a. What are the two central variables in this research, based on what you've read so far?
b. Is this study correlational or experimental?
c. Sketch a graph of the result.
d) The journalist writes:
Now, this study only shows an association between group exercise and improved mental health, and can't prove that the one causes the other.
Why can't the study support that "one causes the other"? Apply the three causal criteria (covariance, temporal precedence, and internal validity) to this study.