. 2013 Jul 3;25:8. doi: 10.1186/2052-4374-25-8
Copyright © 2013 Sung et al.; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Figure 1.
Selection of subjects. * PY: pack-years.