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?

In 2014, the tobacco companies have changed their behavior and have ceased and desisted from arguing that even major differences between cigarette brands render them different in terms of overall public health risk. In fact, Philip Morris has explicitly the following: "As of today ... there is no cigarette on the market that public health organizations endorse as offering 'reduced risk.' If smokers are concerned about the risks of cigarette smoking, the best thing to do is quit."

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