In recent years, a bunch of studies have reported correlations between nicotine vaping and various smoking-related diseases, seemingly undermining the case for e-cigarettes as a harm-reducing alternative to combusted tobacco. But as a new analysis in the journal Internal and Emergency Medicine notes, those studies paid no attention to the question of whether the diseases were diagnosed before or after the subjects began vaping—a glaring omission when you are trying to figure out whether the associations indicate cause-and-effect relationships.