Sumner, W. G., & Keller, A. G. (1914). The challenge of facts, and other essays. Yale University Press.
Lua i Stíl Chicago (17ú heag.)Sumner, William Graham, agus Albert Galloway Keller. The Challenge of Facts, and Other Essays. New Haven: Yale University Press, 1914.
Lua MLA (9ú heag.)Sumner, William Graham, agus Albert Galloway Keller. The Challenge of Facts, and Other Essays. Yale University Press, 1914.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.