On 2017 Feb 06, Clive Bates commented:

To build on to Professor Hajek's cogent criticism, I would like to add a further three points:

First, the authors offer the usual disclaimer that they cannot make causal inferences from a study of this nature, which is correct. But they go on to do exactly that within the same paragraph:

Finally, although the crosssectional design of our study allowed us to determine associations between variables, it restricted our ability to draw definitive causal inferences, particularly about the association between ENDS use and smoking cessation. Nevertheless, the association between ENDS use and attempts at smoking cessation suggests that a substantial proportion of smokers believe that ENDS use will help with smoking cessation. Furthermore, the inverse association between ENDS use and smoking cessation suggests that ENDS use may actually lower the likelihood of smoking cessation. (emphasis added)

From that, they build a policy recommendation:

"Tobacco cessation programs should tell cigarette smokers that ENDS use may not help them quit smoking"

That statement is literally true for e-cigarettes and every other way of quitting smoking, but it is not a meaningful or legitimate conclusion to draw from this study because the design does not allow for causal inferences.

Second, the authors characterise e-cigarette use as 'ever use' in calculating their headline odds ratio (0.53).

Our most important finding was that having ever used ENDS was significantly associated with reduced odds of quitting smoking.

Ever use could mean anything from 'used once and never again' to 'use all day, every day' or 'used once when I couldn't smoke'. What it does not mean is 'used an e-cigarette in an attempt to quit smoking'. So this way of characterising e-cigarette use can tell us little about people who do try to quit smoking using an e-cigarette or whether that approach should be recommended.

Third, as well as the basic timing point made by Professor Hajek, the authors do not consider a further obvious contributory explanation: reverse causality. It is quite possible that those who find it hardest to quit or don't want to quit may be those who are drawn to trying e-cigarettes - either because they don't want to stop or have tried everything else already and failed. It is not safe to assume that the population is homogeneous in the degree of nicotine dependence, that e-cigarette ever-use is randomly distributed across the population or that e-cigarette use is generally undertaken with the intention of quitting.

The analysis provides no insights relevant to the efficacy of e-cigarettes in smoking cessation and building any sort of recommendation to smoking cessation programs based on this survey is wrong and inappropriate.

On 2017 Feb 06, Peter Hajek commented:

The unsurprising finding that people who quit smoking between 2009 and 2013 were less likely to try e-cigarettes than those who still smoked in 2014 is presented as if this shows that the experience with vaping somehow ‘reduced odds of quitting smoking’. It shows no such thing.

It is obvious that current smokers must be more likely to try e-cigarettes than smokers who quit years ago. E-cigarettes were more widely used in 2014 than in previous years. People who quit smoking up to five years earlier would have much less (or even no) opportunities to try vaping, and no reason to do so after they quit. Current smokers in contrast continue to have a good reason to try e-cigarettes, and are having many more opportunities to do so. This provides no information at all about odds of quitting smoking or about whether e-cigarettes are effective or not.

On 2017 Feb 06, Peter Hajek commented:

The unsurprising finding that people who quit smoking between 2009 and 2013 were less likely to try e-cigarettes than those who still smoked in 2014 is presented as if this shows that the experience with vaping somehow ‘reduced odds of quitting smoking’. It shows no such thing.

It is obvious that current smokers must be more likely to try e-cigarettes than smokers who quit years ago. E-cigarettes were more widely used in 2014 than in previous years. People who quit smoking up to five years earlier would have much less (or even no) opportunities to try vaping, and no reason to do so after they quit. Current smokers in contrast continue to have a good reason to try e-cigarettes, and are having many more opportunities to do so. This provides no information at all about odds of quitting smoking or about whether e-cigarettes are effective or not.