What is the impact of an exercise routine on mood regulation in college-age students?

Dependent Variable: mood regulation ( taken by a self report Brief Mood Introspection Scale for instance)

Independent Variable: Exercise frequency (Also self-report, tracking journal)

Quantitative design: 4 week study, Dependent variable will be measured with a pretest posttest with control, one group will engage in 60 minutes of self guided exercise daily. Control will act as if, logging daily activities. Pre test will take place prior to beginning the study, post test will take place after 4 weeks, then a follow up 2 weeks later.

Identify at least one threat to validity or reliability from the study above. Explain how this particular threat might look in the study and how it might impact the findings.