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First, we did not treat either kind of data as "more relaible."  All studies were included in the main meta-analysis.  In addition, the sensitivity analysis (Figure 2) showed that the results from the clinical trials were not significantly different from the observational studies.
The advantage of clinical trials is that they are conducted in highly controlled environments so are easier to interpret. 
The advantage of observational studies is that they reflect actual use patterns in the real world. 
It is well established that medicines do not work as well in the real world because they are often not used or monitored properly or because they are used by people beyond the kind of people who are in the clinical trials.
It is also important to keep in mind that e-cigarettes are not prescription medicines that are used under close medical supervision, but mass marketed consumer products.  For that reason, I think that the observational studies are more relevant to assessing e-cigs.  Having said that, as noted above, we treated both kinds of data equally in the analysis.

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