The availability of electronic cigarettes in US retail outlets, 2012: results of two national studies
Shyanika W Rose, Dianne C Barker, Heather D'Angelo, Tamkeen Khan, Jidong Huang, Frank J Chaloupka, Kurt M Ribisl
Tobacco Control Jul 2014, 23 (suppl 3) iii10-iii16; DOI: 10.1136/tobaccocontrol-2013-051461