The authors of this study make one of the most cardinal errors in all of epidemiology. They ignore the principle that "correlation does not equal causation."
Here, they find a correlation between e-cigarette use and higher and more sustained levels of smoking. But because this is a cross-sectional study, they cannot determine which came first. In other words, what is the direction of the causal relationship? Does the e-cigarette use precede, and cause, the smoking? Or does the smoking precede, and cause, the e-cigarette use?
The problem is that in this cross-sectional study, there is no way to determine the direction of the observed relationship.
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."
Furthermore, later in the paper they reinforce this point more specifically, writing: "the cross-sectional nature of our study does not allow us to identify whether most youths are initiating smoking with conventional cigarettes and then moving on to (usually dual use of) e-cigarettes or vice versa...".
Thus, the authors readily acknowledge that it is impossible from this study to determine whether or not e-cigarettes lead to smoking or whether smoking leads to e-cigarette experimentation.
Nevertheless, this does not stop the authors from drawing a conclusion. They conclude, despite their acknowledged inability to draw such a conclusion, that: "e-cigarette use is aggravating rather than ameliorating the tobacco epidemic among youths."
In other words, despite acknowledging that they cannot tell from their study whether e-cigarette use precedes smoking or whether smoking precedes e-cigarette use, they nonetheless draw the conclusion that e-cigarette use precedes smoking.
The Rest of the Story: Tobacco News Analysis and Commentary: Conclusion of New Glantz Study on Electronic Cigarettes is Junk Science