Santos, Eleonora, Alexandra I. Correia, Fernanda A. Ferreira, Alcina Maria Nunes, Cláudia Miranda Veloso, Elvira Vieira, Goretti Silva, Manuel Fonseca, Paulo Carrança, and Paula Odete Fernandes. 2020. “Productivity Measurement: The Case of Nature Tourism Firms in Portugal”. Journal of Tourism, Sustainability and Well-Being 8 (4):331-46. https://journals.cinturs.pt/jtsw/article/view/252.