A study reported today in The Journal of the American Medical Association claims to present evidence that “e-cigarette use is aggravating rather than ameliorating the tobacco epidemic among youths.” The authors, Lauren Dutra and Stanton Glantz of the Center for Tobacco Research and Education at the University of California in San Francisco, claim their results “suggest that e-cigarettes are not discouraging use of conventional cigarettes.” They add that their findings “call into question claims that e-cigarettes are effective as smoking cessation aids.” But as Boston University public health professor Michael Siegel observes on his tobacco policy blog, Dutra and Glantz “make one of the most cardinal errors in all of epidemiology” by ignoring “the principle that ‘correlation does not equal causation.'”