Christensen T, Riis AH, Hatch EE, Wise LA, Nielsen MG, Rothman KJ, Toft Sorensen H, Mikkelsen EM. Costs and efficiency of online and offline recruitment methods: a web-based cohort study. J Med Internet Res. 2017 Mar 1;19(3):e58. doi: 10.2196/jmir.6716
Whittington D, Sur D, Cook J, Chatterjee S, Maskery B, Lahiri M, Poulos C, Boral S, Nyamete A, Deen J, Ochiai L, Bhattacharya SK. Rethinking cholera and typhoid vaccination policies for the poor: private demand in Kolkata, India. World Dev. 2009 Feb 1;37(2):399-409.