Leffler, G. L. (1952). Seasonal changes in Pennsylvania business.
Lua i Stíl Chicago (17ú heag.)Leffler, George Leland. Seasonal Changes in Pennsylvania Business. State College, 1952.
Lua MLA (8ú heag.)Leffler, George Leland. Seasonal Changes in Pennsylvania Business. 1952.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.