On 2017 Feb 28, Zvi Herzig commented:

To conclude that dual use does not increase cessation, plausible alternative explanations of the data must be excluded, including the very plausible possibility that treatment failures are more resistant to cessation compared to ordinary smokers.

On 2016 Jun 13, Lamberto Manzoli commented:

Well, the distinction between quitting failures and treatment failures, overall and especially in our study, is based upon no data and, as such, it is just a hypothesis. In any case, even if such a distinction would have been "significant", it would not invalidate the main point: in real life, dual users did not quit tobacco more frequently than tobacco only smokers.

On 2016 Jun 10, Zvi Herzig commented:

The authors note that "dual use did not improve the likelihood of quitting tobacco or e-cigarette use". However, dual use indicates a history of resistance to treatment (ie, e-cigarette use without cessation). This severe bias restrains meaningful comparison of cessation-rates between dual users and other smokers.

Indeed, the authors note: "It has been suggested that dual users are frequently ‘quitting failures’ and thus biased against cessation. However, in our sample, the proportion of those who tried quitting before was quite similar between tobacco smokers (33.7%) and dual users (35.9%)."

However, the source quoted for this suggestion (McRobbie H, 2014) mentions 'treatment failures' rather than 'quitting failures'. This is significant: Regardless of whether dual users have failed cessation or not, they have shown resistance to EC-induced cessation. Furthermore, if indeed dual use is not associated with attempted cessation, then dual users have demonstrated a special interest in nicotine and disregard for health, which likely biases against cessation.

On 2016 Jun 10, Zvi Herzig commented:

The authors note that "dual use did not improve the likelihood of quitting tobacco or e-cigarette use". However, dual use indicates a history of resistance to treatment (ie, e-cigarette use without cessation). This severe bias restrains meaningful comparison of cessation-rates between dual users and other smokers.

Indeed, the authors note: "It has been suggested that dual users are frequently ‘quitting failures’ and thus biased against cessation. However, in our sample, the proportion of those who tried quitting before was quite similar between tobacco smokers (33.7%) and dual users (35.9%)."

However, the source quoted for this suggestion (McRobbie H, 2014) mentions 'treatment failures' rather than 'quitting failures'. This is significant: Regardless of whether dual users have failed cessation or not, they have shown resistance to EC-induced cessation. Furthermore, if indeed dual use is not associated with attempted cessation, then dual users have demonstrated a special interest in nicotine and disregard for health, which likely biases against cessation.