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On Friday I exercise for one hour in the morning and record my mood and on Saturday I don't exercise and record my mood. I find that on Saturday I was happier than Friday. Can I conclude that exercise causes me to be less happy? Why or why not?
No because I was probably doing things differently on Friday (a school day) then on Saturday. There are a lot of other variables that could have influenced my mood: coworkers, students, sleep, classroom environment, activities, etc.