Schmitt CL, Allen JA, Kosa KM, Curry LE. Support for a ban on tobacco powerwalls and other point-of-sale displays: findings from focus groups. Health Educ Res. 2015 Feb;30(1):98-106. doi: 10.1093/her/cyu046.
Phanikumar CV, Maitra B. Willingness-to-pay and preference heterogeneity for rural bus attributes. J Transp Eng A-Syst. 2007 Jan;133(1).