1)What is the difference between an "observational study" and an "experimental study"?

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2)A child psychologist wants to find out if playing video games with violent content makes kids more prone to violent behavior. The psychologist has children come into a lab and has a group of children play a highly violent video game (experimental group) and another group of children play a game with no violence (control group). Then the psychologist watches the kids play with other children and records any violent behaviour.

How could this be done as an observational study? Be specific about how you might conduct a similar study.