If this paper weren't biased, it would mention the chief source of evidence in favor of the idea that smoking damages your lungs: Men who smoke die of lung can

If this paper weren't biased, it would mention the chief source of evidence in favor of the idea that smoking damages your lungs: Men who smoke die of lung cancer at 23 times the rate of nonsmokers. Now, that may not be from a randomized trial. But nonetheless, I think that's an important explanandum. See Judea Pearl's Causality if you don't think it's possible to get important data off of mere observational studies. It just seems really unlikely that, you know, future lung cancer would cause people to start smoking now, or that some common cause makes people decide to start inhaling carcinogens and tar and then causes them to get lung cancer later at 23 times the base rate. But if anyone actually believes that, we can get a group of 1000 of them together and have 500 of them start smoking with proper random assignment, and see what happens.