The authors admit this in the paper. They write: "This is a cross-sectional study, which only allows us to identify associations, not causal relationships."

In other words, the blatant flaw in this study is that it enrolled smokers who had already made quit attempts using e-cigarettes in the past. By definition, this resulted in a sample of smokers who were more resistant to quitting. One would expect to find lower quitting rates among this population of e-cigarette users.

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