Introduction:
Evidence on the association between tobacco outlet density (TOD) and smoking behaviour among youth is inconclusive, which may be due to differences in methodological quality. The aim of this systematic review was to examine to what extent the methodological quality of studies determined the association between TOD and smoking behaviour among young people, by 1) assessing potential sources of bias, and 2) identifying differences in methodological quality between studies that found positive, negative, and/or no significant associations.

Methods:
MEDLINE, EMBASE and Google Scholar were systematically searched for studies on TOD and smoking behaviour among young people published between 1997 and 2017. The methodological quality of the included studies was evaluated independently by two reviewers using the NIH Study Quality Assessment Tool for observational studies and the Cochrane Collaboration Risk of Bias assessment.

Results:
Nineteen studies were included in the review. The quality assessment identified five primary sources of bias: misclassification of exposure measurements, over- and under-adjustment for confounders, selection bias, and under-powered analyses. Taking these biases into account, the 12 studies with a positive association were unlikely to have overestimated the association, whereas the 16 studies that did not find significant associations were more likely to have underestimated the association. Although not of poor quality, three studies found negative associations.

Conclusions:
Considering the quality of the evidence, higher TOD is likely to be associated with higher smoking rates. Future research should establish the causality of this association in order to identify whether removing tobacco outlets would lead to lower smoking rates.

Funding:
This study is part of the SILNE-R project, which received funding from the European Commission (EC), Horizon2020 program, Call PHC6-2014, under Grant Agreement n°635056.