Must Try Harder: Evaluating the Role of Effort in Educational Attainment
Gianni De Fraja, Tania Oliveira, Luisa Zanchi
Review of Economics and Statistics, vol. 92, 2010, p. 577-597.

Mixing Goods with Two-Part Tariffs
Steffen Hoernig, Tommaso Valletti
European Economic Review, vol. 51, 2007, p. 1733-1750.