How do we determine the impact of e-cigarettes on cigarette smoking cessation or reduction? Review and recommendations for answering the research question with scientific rigor.

AIMS: To propose a hierarchy of methodological criteria to consider when determining whether a study provides sufficient information to answer the question of whether e-cigarettes can facilitate cigarette smoking cessation or reduction.
DESIGN: A PubMed search to 1 February 2017 was conducted of all studies related to e-cigarettes and smoking cessation or reduction. SETTINGS: Australia, Europe, Iran, Korea, New Zealand and the United States. PARTICIPANTS AND STUDIES: 91 articles. MEASUREMENTS: Coders organized studies according to six proposed methodological criteria: (1) examines outcome of interest (cigarette abstinence or reduction), (2) assesses e-cigarette use for cessation as exposure of interest, (3) employs appropriate control/comparison groups, (4) ensures that measurement of exposure precedes the outcome, (5) evaluates dose and duration of the exposure and (6) evaluates the type and quality of the e-cigarette used.
FINDINGS: Twenty-four papers did not examine the outcomes of interest. Forty did not assess the specific reason for e-cigarette use as an exposure of interest. Twenty papers did not employ prospective study designs with appropriate comparison groups. The few observational studies meeting some of the criteria (duration, type, use for cessation) triangulated with findings from three randomized trials to suggest that e-cigarettes can help adult smokers quit or reduce cigarette smoking.
CONCLUSIONS: Only a small proportion of studies seeking to address the effect of e-cigarettes on smoking cessation or reduction meet a set of proposed quality standards. Those that do are consistent with randomized controlled trial evidence in suggesting that e-cigarettes can help with smoking cessation or reduction.