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 argument being made by the CDC, as well as many other anti-smoking groups and advocates, is that electronic cigarettes are a gateway to youth smoking. In other words, youth who have never smoked before are going to try electronic cigarettes, become addicted to nicotine, and then move on to tobacco cigarette smoking.

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