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The problem is, this isn't always feasible. Take, for example, what I imagine you take as a fairly obvious causal link between smoking cigarettes and developing cancer.I'm a simple man I like Star Wars Avenger and Game of Thrones shirt  You can't ethically assign a random group of people to smoke or not smoke for the rest of their lives and see what happens. And since you can't randomly assign, it could be the case there there is a group of people who are genetically predisposed to lung cancer, and that same predisposition also makes them enjoy cigarettes more, and therefore end up smoking more.
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 Technically, we can't ever get around this, and at times we have to just rely on theory and other studies to fill in the gaps of what reasonable causal relationships are.I'm a simple man I like Star Wars Avenger and Game of Thrones shirt 
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