The problem is that in this cross-sectional study, there is no way to determine the direction of the observed relationship.

What possible justification is there for banning advertising for electronic cigarettes, but not for the real thing?

Some of the most important conclusions in epidemiologic research have come not from large case-control or cohort studies or clinical trials, but from case studies or case series, or as Dr. Frieden would say, "anecdotes."

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