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How often do you smoke marijuana?

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Hermione and Luna conducted a study on the effects of marijuana on academic performance, and the effect of school on marijuana usage. They recruited 50 people from a well-known group of marijuana users at school, and 50 people from a well-known group of abstainers. At the beginning of the school year, they administered three measurements of key variables: (a) marijuana use—“how often do you smoke marijuana?” (b) grade point average (GPA)— “what were your grades for the previous semester?”; and (c) relationship with their teachers— “in general, how good is your relationship with your teachers?” At the end of the school year, the researchers reassessed the three key variables. They found that by the end of the school year, 25% of the users had dropped out of school (and thus were out of the study), whereas none of the abstainers had dropped out. They also found that students in the “users” group had significantly poorer GPAs and poorer relationships with their teachers than those in the “non-user” group. However, they also found that marijuana users decreased their usage by the end of the school year, whereas a significant percentage of abstainers started using marijuana by the end of the school year. Thus, they concluded that marijuana negatively affects students’ scholastic achievements, that school helps habitual users stop using marijuana but simultaneously causes people who never used marijuana to start using. Obviously, this study has many problems, particularly with internal validity. Please describe how four of the eight threats to internal validity may have influenced the results of this study, and offer solutions for how to deal with these internal validity threats. In your answer, be sure to describe each threat, identify how each threat is present in the study, explain how the threat would specifically impact the results of the study, and offer one suggestion for how to solve or prevent each threat.