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Contributors ECL developed the concept for the study and performed the analytic calculations. All authors contributed to the interpretation of the results and the final version of the manuscript.
Funding The work is funded by Grants T31IR1584 and 28IR-0066 from the Tobacco Related Disease Research Program. NS also acknowledges salary support from the National Institute of Mental Health (Grant 5T32MH122376).
Disclaimer The study sponsors took no part in the study design; collection, analyses and interpretation of data; writing of the manuscript; or decision to submit the manuscript for publication.
Map disclaimer The depiction of boundaries on this map does not imply the expression of any opinion whatsoever on the part of BMJ (or any member of its group) concerning the legal status of any country, territory, jurisdiction or area or of its authorities. This map is provided without any warranty of any kind, either express or implied.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.